march: Fix concurrent updates at sector abandon prompt

march: Fix concurrent updates at sector abandon prompt

When the player declines to abandon a sector, we write back stale land
units, triggering a generation oops.  Any updates made by other
threads meanwhile are wiped out, triggering a seqno mismatch oops.

The culprit is lnd_abandon_askyn(): when the player declines, it
returns without calling check_sect_ok(), check_land_ok().  Broken in
commit 7c1b166, v4.3.33.  Fix it.

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