slot which won't leave gaps (in find_free_slot()) would fail if the inventory was empty; fixed bug by making find_free_slot() retry with forwards search if backwards search failed. I can't reproduce the bug, but the fix doesn't seem to cause any problems when there isn't a bug.
git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2368 c06c8d41-db1a-0410-9941-cceddc491573
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