libtracker-miner: Check the crawler cancellable before destroying it
libtracker-miner: Check the crawler cancellable before destroying it

It won't be set unless tracker_crawler_start() was called, so there's a
chance it could be NULL on finalize. Spotted on make distcheck

outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker