Fix clang report failure when no warnings.

Clang report fails when there are no warnings to report.
Report directory is empty (and it gets erased), so `cp` command after `scan-build` fails.

@justinmk I haven't been able to test this, so it may need some minor adjustment. But the idea should work.
Oops, I also noticed the permissions were changed on this file:

    100755 → 100644

I'm not at a unix machine right now so I can't fix this.