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.
`if [ $scan_build_status ] ; then` should be sufficient and idiomatic if I'm not mistaken.