bolt::agent() could crash if beam_source was negative (the beam was caused by a

miscast effect).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7896 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on December 21, 2008
5MNRCFTWZ5CYLK2HCRMZUUMIXEFTI2KQAICVCFLZJ7F4TDN3IF5QC
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