subs: Tighten the check for destroyed defending land unit

subs: Tighten the check for destroyed defending land unit

get_dland() tests efficiency to find out whether the defending land
unit is gone.  Doesn't work when somebody else has since built a new
one with the same UID.  Not sure that can happen.  Check for a change
of owner instead.  Matches get_oland().

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