Fix explosions not affecting the right grids and exploding inside walls.

Don't finish beam if it's reflected by a monster.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7978 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on December 26, 2008
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