mplayer: don't display "-1" as chapter when chapter seek fails

Increase robustness against out of bound chapter numbers. Normally
these functions expect that the callers sanitize the chapter number.
This went wrong at least in add_seek_osd_messages() (which displayed
a chapter "-1" when chapters were not available). Make these functions
a bit friendler and add some reasonable checks and fallbacks, which
fixes the mentioned chapter seeking case as well.