Fixed spell name cache asserting on invalid spells.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@4017 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on April 1, 2008
7KVPF74ACO6Q5FXS2YBBJTJT4Y4YN2M3ZNIPXFI3QSQNGNTHD7AAC
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