Fast Micro AI ai_helper functions: fix rarely occurring bug

When the AI moves a unit out of the way for another unit’s move, it is
occasionally possible for the former unit to use up all its movement
points (e.g. if it has few MP or on narrow crowded maps). If that unit
is on the list to be moved later during the same sequence, this caused
ai_helper.robust_move_and_attack() to return nil, which then made the
Fast Micro AI produce an error message.

data/ai/lua/ai_helper.lua

@@ -329,7 +329,7 @@ function ai_helper.robust_move_and_attack(ai, src, dst, target_loc, cfg)
     end
 
     local gamestate_changed = false
-    local move_result
+    local move_result = ai_helper.dummy_check_action(false, false, 'robust_move_and_attack::NO_ACTION')
     if (unit.moves > 0) then
         if (src_x == dst_x) and (src_y == dst_y) then
             move_result = ai.stopunit_moves(unit)