Crash fix & nonsense mode list fix for legacy monitor types
A fix for a crash that was reported on the forums (https://www.riscosopen.org/forum/forums/5/topics/17705?page=4#posts-140181), and a fix for another issue spotted during testing which was causing Service_EnumerateScreenModes to return mostly nonsense for the legacy monitor types (i.e. not MDF/EDID)