avformat/movenc: Fix segfault upon allocation error

Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/movenc.c

@@ -6254,6 +6254,9 @@ static void mov_free(AVFormatContext *s)
     MOVMuxContext *mov = s->priv_data;
     int i;
 
+    if (!mov->tracks)
+        return;
+
     if (mov->chapter_track) {
         if (mov->tracks[mov->chapter_track].par)
             av_freep(&mov->tracks[mov->chapter_track].par->extradata);