[#2207] Removed unnecessary null check

STACK Output:
bug: anti-dce
model: |
  %tobool130 = icmp ne %"class.irods::auth"* %86, null, !dbg !3786
  -->  true
  ************************************************************
  if.then131:
  call void (i32, i8*, ...)* @rodsLog(i32 3, i8* getelementptr inbounds ([39 x i8]* @.str75, i32 0, i32 0)), !dbg !3788
  br label %if.end132, !dbg !3790
stack:
  - /home/kellerb/software/root-stack/irods/plugins/auth/pam/libpam.cpp:518:0
ncore: 1
core:
  - /home/kellerb/software/root-stack/irods/plugins/auth/pam/libpam.cpp:514:0
    - null pointer dereference