avcodec/pthread_frame: Fix memleak of AVCodecContext on error

Fixes CID1135767
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread_frame.c

@@ -650,6 +650,7 @@ int ff_frame_thread_init(AVCodecContext *avctx)
         p->frame = av_frame_alloc();
         if (!p->frame) {
             err = AVERROR(ENOMEM);
+            av_freep(&copy);
             goto error;
         }