preserve the piety pool when it would cause an overflow, and to increase
the maximum value such that donations won't be lost in a normal game.
I'm not sure what upper limit to set, especially considering that this
is supposed to support DOS and possibly 16-bit values, so, seeing as
durations are signed ints, I've set it to 32767, which you can only get
all at once with a donation of a little less than 500000 gold.
git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8501 c06c8d41-db1a-0410-9941-cceddc491573
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