Fix false-positive completion detection in X Solo.

Fix false-positive completion detection in X Solo.

Spotted by Michael Quevillon. After checking the set of digits
appearing in one of the two main grid diagonals, we weren't clearing
the used[] array before using it to check the same thing about the
other diagonal. So if the first diagonal we check has one of
everything, then the second will be misidentified as correct (for the
purposes of game-completion detection, although not error
highlighting) even if it doesn't have one of everything.