avformat/mov: fix leak of the gain map parameters when the tmap payload allocation fails

The payload size is the item's extent length from the file, so the
allocation can fail for a crafted file.

Leak Replicated through UnModified FFmpeg with LSAN

Found by a grok bot based audit of the code.

libavformat/mov.c

@@ -11662,7 +11662,7 @@ static int mov_parse_heif_layered_video_tmap(AVFormatContext *s, const HEIFGroup
 
     payload = av_malloc(tmap->item->extent_length);
     if (!payload) {
-        return AVERROR(ENOMEM);
+        err = AVERROR(ENOMEM);
         goto fail;
     }