Don't crash if no input filename was specified.
Don't crash if no input filename was specified.

Fixes <rdar://problem/13723332>.

Swift SVN r4986

tkremenek/swift,Jnosh/swift,nathawes/swift,KrishMunot/swift,khizkhiz/swift,allevato/swift,rudkx/swift,kusl/swift,jtbandes/swift,milseman/swift,xwu/swift,tjw/swift,gottesmm/swift,Jnosh/swift,ben-ng/swift,return/swift,austinzheng/swift,alblue/swift,russbishop/swift,ben-ng/swift,djwbrown/swift,hughbe/swift,tinysun212/swift-windows,alblue/swift,nathawes/swift,modocache/swift,tjw/swift,xwu/swift,gmilos/swift,jmgc/swift,KrishMunot/swift,devincoughlin/swift,rudkx/swift,JaSpa/swift,gregomni/swift,JGiola/swift,manavgabhawala/swift,SwiftAndroid/swift,tjw/swift,amraboelela/swift,kstaring/swift,gregomni/swift,jopamer/swift,shajrawi/swift,cbrentharris/swift,swiftix/swift,brentdax/swift,Ivacker/swift,allevato/swift,OscarSwanros/swift,gregomni/swift,hughbe/swift,codestergit/swift,huonw/swift,gribozavr/swift,emilstahl/swift,zisko/swift,huonw/swift,swiftix/swift,gottesmm/swift,dduan/swift,swiftix/swift,sschiau/swift,bitjammer/swift,deyton/swift,kentya6/swift,kperryua/swift,xwu/swift,glessard/swift,johnno1962d/swift,practicalswift/swift,ahoppen/swift,parkera/swift,uasys/swift,dduan/swift,djwbrown/swift,ken0nek/swift,milseman/swift,parkera/swift,lorentey/swift,allevato/swift,tinysun212/swift-windows,devincoughlin/swift,cbrentharris/swift,ahoppen/swift,ken0nek/swift,apple/swift,LeoShimonaka/swift,frootloops/swift,MukeshKumarS/Swift,arvedviehweger/swift,JGiola/swift,jmgc/swift,arvedviehweger/swift,IngmarStein/swift,rudkx/swift,Jnosh/swift,shahmishal/swift,hooman/swift,airspeedswift/swift,JaSpa/swift,CodaFi/swift,mightydeveloper/swift,tardieu/swift,austinzheng/swift,hughbe/swift,kstaring/swift,harlanhaskins/swift,amraboelela/swift,KrishMunot/swift,KrishMunot/swift,return/swift,OscarSwanros/swift,KrishMunot/swift,slavapestov/swift,shahmishal/swift,xedin/swift,CodaFi/swift,ben-ng/swift,manavgabhawala/swift,brentdax/swift,LeoShimonaka/swift,OscarSwanros/swift,felix91gr/swift,apple/swift,modocache/swift,karwa/swift,kentya6/swift,stephentyrone/swift,stephentyrone/swift,jtbandes/swift,Ivacker/swift,Ivacker/swift,russbishop/swift,slavapestov/swift,mightydeveloper/swift,tjw/swift,practicalswift/swift,djwbrown/swift,nathawes/swift,danielmartin/swift,sschiau/swift,slavapestov/swift,SwiftAndroid/swift,austinzheng/swift,gmilos/swift,frootloops/swift,natecook1000/swift,devincoughlin/swift,benlangmuir/swift,benlangmuir/swift,slavapestov/swift,adrfer/swift,tjw/swift,return/swift,aschwaighofer/swift,glessard/swift,zisko/swift,JaSpa/swift,LeoShimonaka/swift,gregomni/swift,benlangmuir/swift,hooman/swift,swiftix/swift.old,ben-ng/swift,danielmartin/swift,johnno1962d/swift,practicalswift/swift,harlanhaskins/swift,kstaring/swift,benlangmuir/swift,danielmartin/swift,russbishop/swift,codestergit/swift,shajrawi/swift,rudkx/swift,swiftix/swift,frootloops/swift,mightydeveloper/swift,swiftix/swift.old,devincoughlin/swift,MukeshKumarS/Swift,airspeedswift/swift,johnno1962d/swift,swiftix/swift.old,hooman/swift,arvedviehweger/swift,nathawes/swift,parkera/swift,kusl/swift,calebd/swift,uasys/swift,shahmishal/swift,felix91gr/swift,roambotics/swift,cbrentharris/swift,brentdax/swift,jmgc/swift,hooman/swift,deyton/swift,kusl/swift,frootloops/swift,natecook1000/swift,manavgabhawala/swift,manavgabhawala/swift,arvedviehweger/swift,brentdax/swift,tinysun212/swift-windows,JaSpa/swift,sschiau/swift,frootloops/swift,JGiola/swift,KrishMunot/swift,return/swift,khizkhiz/swift,ahoppen/swift,austinzheng/swift,alblue/swift,CodaFi/swift,jopamer/swift,mightydeveloper/swift,xedin/swift,codestergit/swift,rudkx/swift,shahmishal/swift,shajrawi/swift,hughbe/swift,jckarter/swift,sschiau/swift,milseman/swift,ken0nek/swift,cbrentharris/swift,apple/swift,sdulal/swift,zisko/swift,therealbnut/swift,gmilos/swift,russbishop/swift,russbishop/swift,deyton/swift,ken0nek/swift,hughbe/swift,rudkx/swift,manavgabhawala/swift,tkremenek/swift,nathawes/swift,kentya6/swift,tkremenek/swift,jopamer/swift,Jnosh/swift,arvedviehweger/swift,tkremenek/swift,sschiau/swift,modocache/swift,deyton/swift,gottesmm/swift,lorentey/swift,glessard/swift,dduan/swift,LeoShimonaka/swift,SwiftAndroid/swift,huonw/swift,djwbrown/swift,slavapestov/swift,emilstahl/swift,jckarter/swift,gribozavr/swift,felix91gr/swift,jtbandes/swift,sdulal/swift,glessard/swift,natecook1000/swift,sschiau/swift,tardieu/swift,atrick/swift,codestergit/swift,djwbrown/swift,deyton/swift,hughbe/swift,emilstahl/swift,natecook1000/swift,CodaFi/swift,kusl/swift,tinysun212/swift-windows,airspeedswift/swift,johnno1962d/swift,xwu/swift,gribozavr/swift,aschwaighofer/swift,dduan/swift,gregomni/swift,nathawes/swift,frootloops/swift,cbrentharris/swift,gottesmm/swift,danielmartin/swift,ken0nek/swift,johnno1962d/swift,khizkhiz/swift,kstaring/swift,brentdax/swift,parkera/swift,manavgabhawala/swift,swiftix/swift.old,modocache/swift,sdulal/swift,swiftix/swift.old,KrishMunot/swift,uasys/swift,roambotics/swift,sschiau/swift,IngmarStein/swift,modocache/swift,zisko/swift,felix91gr/swift,therealbnut/swift,jtbandes/swift,codestergit/swift,allevato/swift,therealbnut/swift,amraboelela/swift,JGiola/swift,amraboelela/swift,SwiftAndroid/swift,ben-ng/swift,CodaFi/swift,aschwaighofer/swift,return/swift,JaSpa/swift,cbrentharris/swift,gribozavr/swift,ben-ng/swift,OscarSwanros/swift,MukeshKumarS/Swift,emilstahl/swift,gribozavr/swift,shajrawi/swift,kperryua/swift,lorentey/swift,gottesmm/swift,gottesmm/swift,calebd/swift,apple/swift,practicalswift/swift,jopamer/swift,emilstahl/swift,practicalswift/swift,modocache/swift,practicalswift/swift,jtbandes/swift,devincoughlin/swift,felix91gr/swift,uasys/swift,kstaring/swift,milseman/swift,djwbrown/swift,bitjammer/swift,adrfer/swift,huonw/swift,sdulal/swift,tardieu/swift,adrfer/swift,jckarter/swift,emilstahl/swift,gmilos/swift,xwu/swift,felix91gr/swift,kperryua/swift,karwa/swift,lorentey/swift,xedin/swift,manavgabhawala/swift,djwbrown/swift,jtbandes/swift,jckarter/swift,harlanhaskins/swift,IngmarStein/swift,atrick/swift,hughbe/swift,jmgc/swift,karwa/swift,tkremenek/swift,kentya6/swift,lorentey/swift,johnno1962d/swift,kusl/swift,jopamer/swift,kusl/swift,xwu/swift,shahmishal/swift,bitjammer/swift,MukeshKumarS/Swift,jopamer/swift,kperryua/swift,codestergit/swift,therealbnut/swift,jckarter/swift,gottesmm/swift,huonw/swift,airspeedswift/swift,dreamsxin/swift,JGiola/swift,kperryua/swift,CodaFi/swift,tinysun212/swift-windows,jopamer/swift,kentya6/swift,russbishop/swift,austinzheng/swift,bitjammer/swift,calebd/swift,hooman/swift,airspeedswift/swift,stephentyrone/swift,bitjammer/swift,kperryua/swift,OscarSwanros/swift,swiftix/swift.old,harlanhaskins/swift,danielmartin/swift,slavapestov/swift,kentya6/swift,jmgc/swift,slavapestov/swift,karwa/swift,gribozavr/swift,tjw/swift,harlanhaskins/swift,tardieu/swift,devincoughlin/swift,parkera/swift,calebd/swift,sschiau/swift,bitjammer/swift,tardieu/swift,ken0nek/swift,felix91gr/swift,therealbnut/swift,roambotics/swift,xedin/swift,shajrawi/swift,IngmarStein/swift,Jnosh/swift,Ivacker/swift,dduan/swift,stephentyrone/swift,devincoughlin/swift,IngmarStein/swift,kentya6/swift,LeoShimonaka/swift,airspeedswift/swift,karwa/swift,johnno1962d/swift,gmilos/swift,parkera/swift,danielmartin/swift,milseman/swift,jckarter/swift,JGiola/swift,atrick/swift,roambotics/swift,aschwaighofer/swift,cbrentharris/swift,austinzheng/swift,calebd/swift,tinysun212/swift-windows,aschwaighofer/swift,benlangmuir/swift,dreamsxin/swift,uasys/swift,tardieu/swift,practicalswift/swift,swiftix/swift,calebd/swift,xedin/swift,hooman/swift,gmilos/swift,karwa/swift,JaSpa/swift,kentya6/swift,airspeedswift/swift,shajrawi/swift,SwiftAndroid/swift,huonw/swift,milseman/swift,mightydeveloper/swift,dduan/swift,milseman/swift,devincoughlin/swift,Jnosh/swift,uasys/swift,jmgc/swift,SwiftAndroid/swift,khizkhiz/swift,deyton/swift,adrfer/swift,khizkhiz/swift,swiftix/swift,Ivacker/swift,gregomni/swift,IngmarStein/swift,mightydeveloper/swift,aschwaighofer/swift,karwa/swift,codestergit/swift,ben-ng/swift,lorentey/swift,roambotics/swift,amraboelela/swift,MukeshKumarS/Swift,kusl/swift,emilstahl/swift,adrfer/swift,hooman/swift,CodaFi/swift,alblue/swift,LeoShimonaka/swift,tkremenek/swift,allevato/swift,SwiftAndroid/swift,return/swift,huonw/swift,sdulal/swift,danielmartin/swift,shajrawi/swift,return/swift,natecook1000/swift,dduan/swift,sdulal/swift,mightydeveloper/swift,brentdax/swift,uasys/swift,IngmarStein/swift,arvedviehweger/swift,zisko/swift,mightydeveloper/swift,amraboelela/swift,modocache/swift,parkera/swift,JaSpa/swift,parkera/swift,kstaring/swift,shahmishal/swift,apple/swift,atrick/swift,zisko/swift,sdulal/swift,russbishop/swift,atrick/swift,deyton/swift,nathawes/swift,natecook1000/swift,calebd/swift,brentdax/swift,adrfer/swift,roambotics/swift,jmgc/swift,glessard/swift,kperryua/swift,ken0nek/swift,stephentyrone/swift,Ivacker/swift,adrfer/swift,LeoShimonaka/swift,shahmishal/swift,xedin/swift,kstaring/swift,tjw/swift,bitjammer/swift,austinzheng/swift,amraboelela/swift,xedin/swift,stephentyrone/swift,shajrawi/swift,cbrentharris/swift,ahoppen/swift,xwu/swift,alblue/swift,benlangmuir/swift,khizkhiz/swift,karwa/swift,LeoShimonaka/swift,MukeshKumarS/Swift,Ivacker/swift,gribozavr/swift,gmilos/swift,allevato/swift,jtbandes/swift,khizkhiz/swift,kusl/swift,frootloops/swift,zisko/swift,aschwaighofer/swift,OscarSwanros/swift,emilstahl/swift,alblue/swift,xedin/swift,practicalswift/swift,jckarter/swift,ahoppen/swift,tinysun212/swift-windows,therealbnut/swift,apple/swift,stephentyrone/swift,arvedviehweger/swift,sdulal/swift,allevato/swift,swiftix/swift,ahoppen/swift,swiftix/swift.old,alblue/swift,harlanhaskins/swift,tardieu/swift,lorentey/swift,MukeshKumarS/Swift,gribozavr/swift,harlanhaskins/swift,therealbnut/swift,Jnosh/swift,glessard/swift,atrick/swift,OscarSwanros/swift,tkremenek/swift,swiftix/swift.old,Ivacker/swift,shahmishal/swift,lorentey/swift,natecook1000/swift