Put in a safety valve for monsters with garbage speed/speed_increment (bug

reported by Erik). We need to figure out how the speed-trashing is happening.

Converted speed and speed_increment to ints to avoid wraparound on zero.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@1229 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on April 6, 2007
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