Add missing null check when parsing subscript expressions.
Add missing null check when parsing subscript expressions.

Swift SVN r15092

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