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.
That's true. But, is this script supposed to be run with -e?
In that case, it should be avoidable embedding the failing command within an if. This is,
```
if scan-build ....
then
   scan_build_status=0
else
   scan_build_status=1
fi
```