pthread_frame: Unreference hw_frames_ctx on per-thread codec contexts

When decoding with threads enabled, the get_format callback will be
called with one of the per-thread codec contexts rather than with the
outer context.  If a hwaccel is in use too, this will add a reference
to the hardware frames context on that codec context, which will then
propagate to all of the other per-thread contexts for decoding.  Once
the decoder finishes, however, the per-thread contexts are not freed
normally, so these references leak.

Merges Libav commit fd0fae60.

Signed-off-by: wm4 <nfxjfg@googlemail.com>
