Clang report: Fix empty report showed when warnings present.

Problem  : Empty error is being showed even if there are warnings.
Cause    : scan-build --status-bugs is working, but checked status is
           that of the pipe, not scan-build's one.
Solution : Set option `pipefail` so that pipe status is that of the last
           failing command.
Merge pull request #40 from elmart/clang-report-not-showing

Fix clang empty report showed when warnings present.