unset Track sets, relying in AVStream->priv_data and will be freed in av_close_input_file

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@14016 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/mxf.c

@@ -1015,6 +1015,9 @@ static int mxf_read_close(AVFormatContext *s)
         case MaterialPackage:
             av_freep(&((MXFPackage *)mxf->metadata_sets[i])->tracks_refs);
             break;
+        case Track:
+            mxf->metadata_sets[i] = NULL; /* will be freed later */
+            break;
         default:
             break;
         }