Don't try to run solver_killer_sums on a Killer cage with only one

Don't try to run solver_killer_sums on a Killer cage with only one
square. It isn't equipped for it, and will try to handle it with the
4-square case and get confused. This can come up if the
DIFF_KINTERSECT pass before that split a cage, and will cause the
solver to miss valid solutions; e.g. 3x3kadu#802065940985372 would
generate an ambiguous puzzle before this change.

[originally from svn r9402]