tools: address possible non-null terminated filenames

tools: address possible non-null terminated filenames

If a filename is a multiple of 18 characters, there will be no null-terminator.
This will result in an invalid access by the constructed StringRef.  Add a test
case to exercise this and fix that handling.  Address this same vulnerability in
llvm-readobj as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206145 91177308-0d34-0410-b5e6-96231b3b80d8