Fix invalid_monster_index(). First, ANON_FRIENDLY_MONSTER is negative,

so it doesn't need to be explicitly checked for. Second, make sure that the test in the function isn't backwards (oops).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@3762 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on March 20, 2008
H7W5Q3C6V33ANX3BSCWSUYPG5XUFDMOCHZ7BZIL7JEWGI74V3LOQC
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