Do not try to read total disc or track number if data atom is too short.

Fixes ticket #659.

libavformat/mov.c

@@ -87,9 +87,10 @@ static int mov_metadata_track_or_disc_number(MOVContext *c, AVIOContext *pb,
 {
     char buf[16];
 
-    short current, total;
+    short current, total = 0;
     avio_rb16(pb); // unknown
     current = avio_rb16(pb);
+    if (len >= 6)
         total = avio_rb16(pb);
     if (!total)
         snprintf(buf, sizeof(buf), "%d", current);