Prefer av_freep() over av_free() for variables in the context for safety.

Originally committed as revision 16838 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/4xm.c

@@ -330,7 +330,7 @@ static int fourxm_read_close(AVFormatContext *s)
 {
     FourxmDemuxContext *fourxm = s->priv_data;
 
-    av_free(fourxm->tracks);
+    av_freep(&fourxm->tracks);
 
     return 0;
 }