fixed one of two bugs that lead to infinite loop in AI Movement code,

...when AI tries to move A->A onimpassable terrain (happens on Dark
Forecast)

src/ai/actions.cpp

@@ -394,6 +394,7 @@ bool move_result::test_route(const unit &un, const team &my_team, const unit_map
 			set_error(E_EMPTY_MOVE);
 			return false;
 		}
+		return true;
 	}
 	const shortest_path_calculator calc(un, my_team, units, teams,map);