Add pointless initialisations to placate a gcc warning.

(I'm confident these can't happen. maxb is initialised whenever we
break from the first loop with y < h, and when we don't break from
that loop the second loop which uses maxb is run zero times. But gcc
can't work that out, sigh.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9546 cda61777-01e9-0310-a592-d414129be87e