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

Such as when the unit is surrounded.

changelog

@@ -21,6 +21,7 @@ Version 1.13.0-dev:
      some minor and subtle bugs.
    * 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
  * Campaigns:
    * Changed all occurrences of {FLAG_VARIANT ragged} to {FLAG_VARIANT6 ragged}
    * Eastern Invasion:

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