Fix: Prevent small memory leak in VSE indexer

We need to unref the packet to tell ffmpeg it is ok to free it after
use.

source/blender/imbuf/intern/indexer.c

@@ -1026,6 +1026,7 @@ static int index_rebuild_ffmpeg(FFmpegIndexBuilderContext *context,
         index_rebuild_ffmpeg_proc_decoded_frame(context, next_packet, in_frame);
       }
     }
+    av_packet_unref(next_packet);
   }
 
   /* process pictures still stuck in decoder engine after EOF