Fix NULL pointer dereference when scroll-wheeling in the editor

From cd8c83532b0d33a3a2e24d9af04dac0455a84625:

  gui::slider* s = gui().find_slider("map-zoom-slider");
  if (point_in_rect(event.x, event.y, s->location())) {
      scrollx = 0; scrolly = 0;
  }

s is not guaranteed to not be NULL. display::find_slider() WILL return
NULL if it can't find the requested widget. As it turns out, the editor
currently lacks a slider called "map-zoom-slider".