I am again gormless! When I overhauled Solo's grid generator in

r6160, I completely failed to ensure that generated grids were _at
most_ the required difficulty. It appears to have been only random
chance that prevented a request for a Trivial puzzle from producing
Extreme. Here's a one-line fix.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6298 cda61777-01e9-0310-a592-d414129be87e