player: dvdnav: fix start time when entering and leaving menu

Unfortunately, quite a hack, because we have check the nav state
outside of discnav.c.

player/misc.c

@@ -106,6 +106,10 @@ double get_start_time(struct MPContext *mpctx)
     struct demuxer *demuxer = mpctx->demuxer;
     if (!demuxer)
         return 0;
+    // We reload the demuxer on menu transitions; don't make it use the first
+    // timestamp it finds as start PTS.
+    if (mpctx->nav_state)
+        return 0;
     return demuxer_get_start_time(demuxer);
 }