[RemoteMirror] Add a missing NULL check when reading metadata in readMetadataAndValueErrorExistential.
[RemoteMirror] Add a missing NULL check when reading metadata in readMetadataAndValueErrorExistential.

rdar://problem/49246601
rudkx/swift,xwu/swift,gribozavr/swift,allevato/swift,nathawes/swift,tkremenek/swift,ahoppen/swift,airspeedswift/swift,shahmishal/swift,aschwaighofer/swift,parkera/swift,karwa/swift,sschiau/swift,harlanhaskins/swift,atrick/swift,shajrawi/swift,CodaFi/swift,gribozavr/swift,tkremenek/swift,sschiau/swift,gregomni/swift,harlanhaskins/swift,shahmishal/swift,devincoughlin/swift,ahoppen/swift,stephentyrone/swift,allevato/swift,tkremenek/swift,benlangmuir/swift,gregomni/swift,rudkx/swift,xwu/swift,jckarter/swift,aschwaighofer/swift,tkremenek/swift,sschiau/swift,harlanhaskins/swift,glessard/swift,atrick/swift,glessard/swift,hooman/swift,tkremenek/swift,xwu/swift,shahmishal/swift,nathawes/swift,harlanhaskins/swift,gregomni/swift,xedin/swift,parkera/swift,harlanhaskins/swift,parkera/swift,allevato/swift,gregomni/swift,rudkx/swift,xwu/swift,parkera/swift,jmgc/swift,roambotics/swift,JGiola/swift,devincoughlin/swift,ahoppen/swift,JGiola/swift,sschiau/swift,apple/swift,xwu/swift,parkera/swift,ahoppen/swift,stephentyrone/swift,allevato/swift,lorentey/swift,devincoughlin/swift,shahmishal/swift,airspeedswift/swift,JGiola/swift,jckarter/swift,gregomni/swift,CodaFi/swift,jckarter/swift,benlangmuir/swift,karwa/swift,lorentey/swift,nathawes/swift,benlangmuir/swift,xedin/swift,nathawes/swift,shajrawi/swift,tkremenek/swift,sschiau/swift,shahmishal/swift,jmgc/swift,glessard/swift,xwu/swift,stephentyrone/swift,stephentyrone/swift,xedin/swift,aschwaighofer/swift,aschwaighofer/swift,jckarter/swift,CodaFi/swift,xedin/swift,shajrawi/swift,gribozavr/swift,shajrawi/swift,allevato/swift,karwa/swift,stephentyrone/swift,gribozavr/swift,glessard/swift,sschiau/swift,glessard/swift,airspeedswift/swift,xedin/swift,parkera/swift,parkera/swift,nathawes/swift,lorentey/swift,gregomni/swift,lorentey/swift,shahmishal/swift,apple/swift,xedin/swift,shajrawi/swift,ahoppen/swift,jckarter/swift,hooman/swift,benlangmuir/swift,hooman/swift,atrick/swift,benlangmuir/swift,devincoughlin/swift,karwa/swift,apple/swift,jckarter/swift,jckarter/swift,airspeedswift/swift,shajrawi/swift,devincoughlin/swift,sschiau/swift,atrick/swift,roambotics/swift,jmgc/swift,aschwaighofer/swift,jmgc/swift,jmgc/swift,hooman/swift,stephentyrone/swift,CodaFi/swift,lorentey/swift,lorentey/swift,aschwaighofer/swift,apple/swift,tkremenek/swift,gribozavr/swift,roambotics/swift,harlanhaskins/swift,jmgc/swift,harlanhaskins/swift,atrick/swift,aschwaighofer/swift,atrick/swift,apple/swift,devincoughlin/swift,CodaFi/swift,shajrawi/swift,roambotics/swift,JGiola/swift,allevato/swift,JGiola/swift,gribozavr/swift,devincoughlin/swift,karwa/swift,CodaFi/swift,roambotics/swift,gribozavr/swift,jmgc/swift,airspeedswift/swift,shahmishal/swift,karwa/swift,gribozavr/swift,hooman/swift,nathawes/swift,lorentey/swift,glessard/swift,sschiau/swift,xedin/swift,hooman/swift,shajrawi/swift,airspeedswift/swift,stephentyrone/swift,airspeedswift/swift,allevato/swift,nathawes/swift,ahoppen/swift,xedin/swift,JGiola/swift,rudkx/swift,hooman/swift,rudkx/swift,roambotics/swift,lorentey/swift,apple/swift,benlangmuir/swift,CodaFi/swift,xwu/swift,shahmishal/swift,karwa/swift,rudkx/swift,karwa/swift,devincoughlin/swift,parkera/swift