pthreads_frame: Do not leak on failure path

CC: libav-stable@libav.org
Bug-Id: CID 1135767
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavcodec/pthread_frame.c

@@ -601,6 +601,7 @@ int ff_frame_thread_init(AVCodecContext *avctx)
 
         p->frame = av_frame_alloc();
         if (!p->frame) {
+            av_freep(&copy);
             err = AVERROR(ENOMEM);
             goto error;
         }