tools: fix heap-buffer-overrun detected via ASAN

Once the auxiliary fields relating to the filename have been inspected, any
following auxiliary fields need not be visited as they have been consumed (the
following fields comprise the filepath as a single unit).

Adjust the test to catch this even if ASAN is not enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206190 91177308-0d34-0410-b5e6-96231b3b80d8