Fix a bug in monster summoning that was causing crashes, for example

when attempting to cast Shadow Creatures in a bazaar (which is bound to fail, anyway). I guess it would have also caused crashes anywhere else where the level was devoid of monsters and creating one failed - for whatever reason. I don't actually know how likely that combination is.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6151 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on June 26, 2008
KABMEHHPUS7RLDZ2A5ADC6GMYSBJ5OHD3WPUBDWBM22XWHB7X6WQC
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