Comment 5 for bug 1090888

This patch works around the problem by moving the database query to the top of the function.
If the query fails, it returns without doing anything else.
This should take care of the crash.

A further step would be to make the library-rescan dialog modal, so that the Mixxx window can't be used while it's running, but that'll take some further discussion with the main Mixxx developers. For now, this patch should be enough.