Add additional safeguards

Co-authored-by: Voyager1 voyager@xbmc.org

Another low hanging fruit, just some small safeguards we have had for a long time. Won't likely fix anything but at least make the lib handling a bit more robust:

• Checks for nullptr in dvdnav_get_restrictions
• Avoids decreasing the usage_count in the hypothetical case the code path gots there and its value is 0 or below