pthreads: reset got_frames on flush.

This fixes memory corruption when seeking in broken streams.
a random mpeg4 in nut file was used to debug.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread.c

@@ -731,6 +731,7 @@ static void park_frame_worker_threads(FrameThreadContext *fctx, int thread_count
                 pthread_cond_wait(&p->output_cond, &p->progress_mutex);
             pthread_mutex_unlock(&p->progress_mutex);
         }
+        p->got_frame = 0;
     }
 }