avoid crash when thrown potion hits bars before a monster

avoid crash when thrown potion hits bars before a monster
From a bug report. <Someone> as slashem-Bugs-883643 on 1/24/2004.  To avoid
using the possibly invalid object pointer after calling bhit(), changed as
suggested to add another level of indirection allowing bhit to null the
object pointer before returning.  Callers that are affected update their
object pointers after bhit returns.