[Freeciv-Dev] (PR#6219) bug in city_can_be_built_here usage

["Jason Short" <jshort@xxxxxxxxxxxxxx>]

In hut_get_city there is a call to city_can_be_built_here.  This passes in
the punit parameter.  But the punit is not the unit city_can_be_built_here
is expecting: the unit is not actually building the city, just finding it.
I don't think this can cause any problems in the current ruleset, but
under some future ruleset it might.

The unit passed in here should be NULL, and this case should be checked
for in city_can_be_built_here.

On a related note I think city_can_be_built_here should eventually check
punit->transported_by rather than comparing movement type to terrain type.

jason