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.

libavcodec/pthread_frame.c

@@ -571,6 +571,8 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
             av_freep(&p->avctx->slice_offset);
         }
 
+        av_buffer_unref(&p->avctx->hw_frames_ctx);
+
         av_freep(&p->avctx->internal);
         av_freep(&p->avctx);
     }