Third argument to dgn.grid() is optional. Fixed error that happened

in placing timed messaging markers (including labyrinth entrances).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7525 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on November 21, 2008
RPP2JHUJDTD7XFHCUPPRE7FX6JZBSTJ5IOBW37LN53IMGCO6DU3AC
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