Slightly improve melee_attack::chaos_affects_defender()'s use of fire_beam().

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7706 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on December 1, 2008
HF3CSMAKAOHWJA6CU4PTZSM6G7LVUV5MHE4ZWGEHCCNRBU43UUUQC
Change contents