[#2207] Separated broken null check into two parts

STACK Output:
bug: anti-dce
model: |
  %cmp = icmp eq i8* %add.ptr, null, !dbg !1566
  -->  false
  ************************************************************
  if.then:
  %status1 = getelementptr inbounds %struct.RuleExecInfo* %rei, i32 0, i32 0, !dbg !1570
  %4 = icmp eq %struct.RuleExecInfo* %rei, null
  call void @opt.bugon(i1 %4), !dbg !1570, !bug !1572
  store i32 -31000, i32* %status1, align 4, !dbg !1570
  br label %return, !dbg !1573
stack:
  - /home/kellerb/software/root-stack/irods/iRODS/server/re/src/sysBackupMS.cpp:49:0
ncore: 1
core:
  - /home/kellerb/software/root-stack/irods/iRODS/server/re/src/sysBackupMS.cpp:48:0
    - pointer overflow