fix #H4428 - mon vs mon thrown cockatrice egg

fix #H4428 - mon vs mon thrown cockatrice egg

There was no code in ohitmon() (for object thrown or launched at a
monster by someone or something other than the hero) to handle an egg
hitting a monster.  Cockatrice egg is monsters' preferred missile,
but if one hit a monster instead of hero it just did minimal damage
without any chance of the side-effect that makes it be preferred.