[Analysis] Un-silence -Wtautological-unsigned-zero-compare in null-deref-ps.c

Stage-2 builds failed:

error: 'warning' diagnostics expected but not seen:
  File /home/buildbot/modules-slave-2/clang-x86_64-linux-selfhost-modules-2/llvm.src/tools/clang/test/Analysis/null-deref-ps.c Line 238: always true
llvm-svn: 315622