lavc: free buffer_frame/pkt on avcodec_open2() failure

Fixes memory leaks in some FATE tests.

libavcodec/utils.c

@@ -1199,6 +1199,8 @@ FF_ENABLE_DEPRECATION_WARNINGS
     av_freep(&avctx->priv_data);
     if (avctx->internal) {
         av_frame_free(&avctx->internal->to_free);
+        av_frame_free(&avctx->internal->buffer_frame);
+        av_packet_free(&avctx->internal->buffer_pkt);
         av_freep(&avctx->internal->pool);
     }
     av_freep(&avctx->internal);