lavc: use buf[0] instead of data[0] as the indicator of an allocated frame

data[0] may be NULL for valid frames with hwaccel pixel formats.

libavcodec/utils.c

@@ -1416,7 +1416,7 @@ int attribute_align_arg avcodec_decode_video2(AVCodecContext *avctx, AVFrame *pi
 
         emms_c(); //needed to avoid an emms_c() call before every return;
 
-        if (ret < 0 && picture->data[0])
+        if (ret < 0 && picture->buf[0])
             av_frame_unref(picture);
 
         if (*got_picture_ptr) {