plugins/lua/zone.lua fix: overlay doesn't run when invalid pasture

While the overlay is processing all animals to see where they're
assigned, if it encounters an animal assigned to a nonexistent
civzone or to a building that isn't a civzone, it errors out.
This patch makes the overlay more robust in these cases.
Note that the overlay will not display the relevant animal, nor
will it attempt to repair the invalid assignment.

Author: SilasD

plugins/lua/zone.lua

@@ -989,7 +989,9 @@ local function get_zone_status(unit_or_vermin, bld_assignments)
             return PASTURE_STATUS.ASSIGNED_HERE.value
         else
             local civzone = df.building.find(assigned_zone_ref.building_id)
-            if civzone.type == df.civzone_type.Pen then
+            if not civzone or not df.building_civzonest:is_instance(civzone) then
+                return PASTURE_STATUS.NONE.value
+            elseif civzone.type == df.civzone_type.Pen then
                 return PASTURE_STATUS.PASTURED.value
             elseif civzone.type == df.civzone_type.Pond then
                 return PASTURE_STATUS.PITTED.value
@@ -1150,7 +1152,9 @@ local function get_cage_status(unit_or_vermin, bld_assignments)
     local assigned_zone_ref = get_general_ref(unit_or_vermin, df.general_ref_type.BUILDING_CIVZONE_ASSIGNED)
     if assigned_zone_ref then
         local civzone = df.building.find(assigned_zone_ref.building_id)
-        if civzone.type == df.civzone_type.Pen then
+        if not civzone or not df.building_civzonest:is_instance(civzone) then
+            return CAGE_STATUS.NONE.value
+        elseif civzone.type == df.civzone_type.Pen then
             return CAGE_STATUS.PASTURED.value
         elseif civzone.type == df.civzone_type.Pond then
             return CAGE_STATUS.PITTED.value