Lurkers Micro AI: fix bug in wander terrain selection

Need to verify that the goal hex for the wander is not occupied by an
ally, otherwise the unit might end up on the wrong type of terrain.

changelog

@@ -9,6 +9,7 @@ Version 1.11.17+dev:
  * Miscellaneous and bug fixes:
    * Bug fix: another instance of "overlapping messages", this time with
      the planning mode activiation hotkey
+   * Lurkers Micro AI: fix bug in wander terrain selection
 
 Version 1.11.17:
  * Add-ons server:

data/ai/micro_ais/cas/ca_lurkers.lua

@@ -73,6 +73,15 @@ function ca_lurkers:execution(ai, cfg)
             })
         reachable_wander_terrain:inter(reach)
 
+        -- Need to restrict that to reachable and not occupied by an ally (except own position)
+        local reachable_wander_terrain = reachable_wander_terrain:filter(function(x, y, v)
+            local occ_hex = wesnoth.get_units {
+                x = x, y = y,
+                { "not", { x = lurker.x, y = lurker.y } }
+            }[1]
+            return not occ_hex
+        end)
+
         if (reachable_wander_terrain:size() > 0) then
             local dst = reachable_wander_terrain:to_stable_pairs()
             local rand = math.random(1, reachable_wander_terrain:size())