will now assert.
git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8627 c06c8d41-db1a-0410-9941-cceddc491573
I7UJOFIGYXAARJDKQFIMOAAWM4CYO5ZRDDW6UWREGCMWMIGM25YQC
TGJZXTUIAKCFZQJ54ZQEBGFBVZSJCAX6AWDRSH3TP7UJRLGUM5SAC
K2CS6TCX2NDVL2ASEHGP4J4K4IJ6FP3ANNKTSIWVG43HPYSBX6ZQC
ASSERT(in_bounds(p));