Fix NULL pointer dereference when middle-click-scrolling in the editor

From commits 36b6b60beb507a1277de59691da051d4a2f75171 and
d04f6af930b18a740c7fccd0ee91d0e635f288e5 (paraphrasing):

  map_location original_loc = resources::controller->get_mouse_handler_base().get_scroll_start();
  .
  .
  .
  if (point_in_rect(mousex, mousey,rect) && resources::controller->get_mouse_handler_base().scroll_started()) {
  .
  .
  .

When in the game map editor, resources::controller is set to NULL since
it is only intended to refer to a play_controller object. But there's no
need to look specifically for a play_controller in this code or get the
current pointer to a controller_base subclass since the pointer we need
is actually none other than 'this', which is always non-NULL.

(get_mouse_handler_base() is a method of the same class the faulty
handle_scroll() method is, controller_base. There are no derived classes
of it other than play_controller and editor_controller, and there is
only supposed to be one such instantiated at a time, so the fix is
trivial.)