Fix #1516: units created with [move_units_fake] disappear between steps

The code accidentally added a wrong fake_unit_ptr to the fake unit manager.
The wrong pointer was removed from the manager immediately after being
added, and therefore only the other fake_unit_ptr, created by unit_mover,
remained. Thus, moving fake units were drawn, but stationary ones weren't.

changelog

@@ -4,6 +4,9 @@ Version 1.13.11+dev:
    * Fixed a few cases of data not displaying in the MP Join Game screen.
    * Fixed required addon names not displaying properly when joining an
      MP game.
+ * WML engine:
+   * Fixed units shown with [move_units_fake] disappearing between steps
+     (bug #1516).
  * Miscellaneous and bug fixes:
    * Fixed standing animation toggle not taking immediate effect (bug
      #1653).

src/game_events/action_wml.cpp

@@ -426,7 +426,7 @@ WML_HANDLER_FUNCTION(move_units_fake,, cfg)
 		DBG_NG << "Path " << paths.size() - 1 << " has length " << paths.back().size() << '\n';
 
 		u->set_location(paths.back().front());
-		u.place_on_fake_unit_manager(resources::fake_units);
+		units.back().place_on_fake_unit_manager(resources::fake_units);
 	}
 
 	LOG_NG << "Units placed, longest path is " << longest_path << " long\n";