Galaxies: fix assertion failure when adding out-of-bounds association.

Adding an association with an out-of-bounds square (i.e. by pressing Return
with a dot selected, and then moving the cursor so the `opposite' arrow was
off the screen) would cause space_opposite_dot() to return NULL, in turn
causing ok_to_add_assoc_with_opposite_internal() to return false, failing
the assertion.

This assertion appears to have been introduced in 68363231.