Fix cargo list shutdown oops

Fix cargo list shutdown oops

EF_PLANE is closed before EF_LAND: if a land unit carries a plane, the
plane goes away before its carrier, and unit_onresize() oopses.  Fix
by not checking cargo list consistency there when the file is already
gone.

unit_cargo_init() has a similar issue, at least theoretically: it
rebuilds cargo lists one after the other.  Zap them all first.