Fix for a race condition in process_file(). If a hash was not in Redis when process_file() was executed, processing would fail without deleting the hash from the database.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
Better fix for MediaCrush/MediaCrush#611.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
Merge pull request #611 from d7415/master

Fix for a race condition in process_file().