Return Guardian MAI: fix bug when path to return hex is blocked

Such as when the unit is surrounded.

changelog

@@ -67,6 +67,7 @@ Version 1.11.15+dev:
      incorrectly set more helpful.
    * Fix bug in Random Recruit Micro AI: the AI can now handle custom castle
      terrain independent of its terrain code
+   * Fix bug in Return Guardian Micro AI when there is no path to the return hex
 
 Version 1.11.15:
  * Graphics:

data/ai/micro_ais/cas/ca_return_guardian.lua

@@ -35,6 +35,8 @@ function ca_return_guardian:execution(ai, cfg)
     end
 
     local nh = AH.next_hop(guardian, x, y)
+    if (not nh) then nh = { guardian.x, guardian.y } end
+
     AH.movefull_stopunit(ai, guardian, nh)
 end