tests/check/elements/videotestsrc.c: Returning a return value often helps. In this case, we don't need the return val...

Original commit message from CVS:
* tests/check/elements/videotestsrc.c: (check_rgb_buf):
Returning a return value often helps. In this case, we
don't need the return value anyway, so just get rid of it.
Should make build bots much happier.