BR 1972213: Move "Corpse x no longer valid!" into #DEBUG_DIAGNOSTICS

        and print "The corpse you are butchering rises to attack you."
        where that is the case.

BR 1945387: Don't allow summoning earth elementals from walls near the edge of the map (that can't get destroyed that way). Just seems more consistent. There's an error message, but it doesn't cost a turn.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5505 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on June 6, 2008
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