[Demangler] Null-check a pointer before using it in arithmetic
[Demangler] Null-check a pointer before using it in arithmetic

This addresses a UBSan -fsanitize=null diagnostic seen when compiling
the stdlib:

runtime error: applying non-zero offset 128 to null pointer

parkera/swift,apple/swift,jmgc/swift,xwu/swift,gregomni/swift,glessard/swift,parkera/swift,roambotics/swift,jmgc/swift,benlangmuir/swift,glessard/swift,gregomni/swift,ahoppen/swift,parkera/swift,roambotics/swift,tkremenek/swift,glessard/swift,hooman/swift,apple/swift,gregomni/swift,hooman/swift,rudkx/swift,JGiola/swift,xwu/swift,gregomni/swift,hooman/swift,tkremenek/swift,benlangmuir/swift,hooman/swift,jmgc/swift,apple/swift,ahoppen/swift,gregomni/swift,JGiola/swift,glessard/swift,hooman/swift,roambotics/swift,parkera/swift,rudkx/swift,atrick/swift,xwu/swift,ahoppen/swift,rudkx/swift,roambotics/swift,ahoppen/swift,parkera/swift,JGiola/swift,tkremenek/swift,hooman/swift,tkremenek/swift,xwu/swift,rudkx/swift,parkera/swift,tkremenek/swift,jmgc/swift,gregomni/swift,atrick/swift,tkremenek/swift,xwu/swift,tkremenek/swift,benlangmuir/swift,jmgc/swift,rudkx/swift,apple/swift,xwu/swift,parkera/swift,atrick/swift,benlangmuir/swift,ahoppen/swift,glessard/swift,apple/swift,apple/swift,ahoppen/swift,xwu/swift,glessard/swift,jmgc/swift,JGiola/swift,benlangmuir/swift,atrick/swift,JGiola/swift,atrick/swift,parkera/swift,atrick/swift,roambotics/swift,rudkx/swift,benlangmuir/swift,jmgc/swift,JGiola/swift,hooman/swift,roambotics/swift