[#2207] Removed unnecessary null check

STACK Output:
bug: anti-dce
model: |
  %tobool = icmp ne %class.pam_auth_plugin* %7, null, !dbg !3773
  -->  true
  ************************************************************
  if.then:
  call void (i32, i8*, ...)* @rodsLog(i32 3, i8* getelementptr inbounds ([49 x i8]* @.str74, i32 0, i32 0)), !dbg !3775
  br label %return, !dbg !3777
stack:
  - /home/kellerb/software/root-stack/irods/plugins/auth/pam/libpam.cpp:500:0
ncore: 1
core:
  - /home/kellerb/software/root-stack/irods/plugins/auth/pam/libpam.cpp:481:0
    - null pointer dereference