Station Guardian MAI: bug fix for unreachable stations

Keep guardian in place if the station is unreachable.

changelog

@@ -4,6 +4,8 @@ Version 1.12.0+dev:
      files.
  * Language and i18n:
    * Updated translations: Portuguese
+ * Miscellaneous and bug fixes:
+   * Stationed Guardian Micro AI: bug fix for unreachable stations
  * WML engine:
    * Fixed nested macros emitting incorrect bindings to the default 'wesnoth'
      textdomain at the end of a substitution instead of the parent textdomain,

data/ai/micro_ais/cas/ca_stationed_guardian.lua

@@ -97,6 +97,10 @@ function ca_stationed_guardian:execution(ai, cfg)
     -- If no enemy is within the target zone, move toward station position
     else
         local nh = AH.next_hop(guardian, cfg.station_x, cfg.station_y)
+        if not nh then
+            nh = { guardian.x, guardian.y }
+        end
+
         AH.movefull_stopunit(ai, guardian, nh)
     end