Filter out SPELL_NO_SPELL from fleeing monster decisions (redundant in trunk, but the checks do no harm). seekspell now asserts when it's asked for a bad spell.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@4015 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on April 1, 2008
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