libtracker-miner: Add missing ref to SPARQL buffer task
On error paths, we let tracker_sparql_buffer_push_finish() still return the underlying TrackerTask, together with the error, so we can detach it properly.
However, in this situation we are not adding an extra task ref, while the callback expects to own a reference. This causes refcounting issues and eventually invalid reads.
Fixes: #130 (closed)