Fix build failure on MacOS by initialising a variable which was

reported as potentially-unused. (In fact, as far as I can tell, it's
only ever uninitialised in assertion-failing code paths, so not a
real bug.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@8873 cda61777-01e9-0310-a592-d414129be87e