When a monster is given an ME_ALERT event concerning it's current foe, update

the monster's target to the foe's new position.

A few more assertions about a monster's target during behaviour_event() and _handle_behaviour().

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8498 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on January 17, 2009
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