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.
I just noticed this. Whitespace within conditional brackets is *required* in bash, so this is likely to fail or cause weird behavior. It should be:

    if [[ "$scan_build_result" == "warnings" ]]; then