demux/cue: fix crash when there are no tracks

demux/cue.c

@@ -259,6 +259,8 @@ struct cue_file *mp_parse_cue(struct bstr data)
 
 int mp_check_embedded_cue(struct cue_file *f)
 {
+    if (f->num_tracks == 0)
+        return -1;
     char *fn0 = f->tracks[0].filename;
     for (int n = 1; n < f->num_tracks; n++) {
         char *fn = f->tracks[n].filename;

demux/demux.c

@@ -3188,7 +3188,7 @@ static void demux_init_cuesheet(struct demuxer *demuxer)
     struct cue_file *f = mp_parse_cue(bstr0(cue));
     if (f) {
         if (mp_check_embedded_cue(f) < 0) {
-            MP_WARN(demuxer, "Embedded cue sheet references more than one file. "
+            MP_WARN(demuxer, "Embedded cue sheet references zero or multiple files. "
                     "Ignoring it.\n");
         } else {
             for (int n = 0; n < f->num_tracks; n++) {