retreat: Fix stack smash in land unit group retreat

retreat: Fix stack smash in land unit group retreat

retreat_land() reads ships instead of land units, overrunning local
variable land.  On lucky systems such as mine, this clobbers ni, and
triggers an oops.  On unlucky systems, it crashes.  On really unlucky
systems, it corrupts the land units file.

Broken since land unit retreat was added in Chainsaw 3.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>