VLCThumbnailer: Fix inverted logic.
VLCThumbnailer: Fix inverted logic.

While this was working fine, it was actually not waiting for a proper
success signaling. This was clearly a race condition, as was
jeopardizing our error/timeout detection

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary