avformat/mmst: Use av_freep() avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mmst.c

@@ -477,8 +477,8 @@ static int mms_close(URLContext *h)
     }
 
     /* free all separately allocated pointers in mms */
-    av_free(mms->streams);
-    av_free(mms->asf_header);
+    av_freep(&mms->streams);
+    av_freep(&mms->asf_header);
 
     return 0;
 }