This fixes a crash when the opacity of a not-visible drawin is changed. This
happened because we got a PropertyNotify for our own change of _NET_WM_OPACITY
and then tried to set the drawin's opacity to this new value.
However, the drawin was only reachable through globalconf.drawins and wasn't
ref'd in lua. This means that the luaA_object_push() call in
property_handle_net_wm_opacity() didn't find the drawin and instead pushed a
nil. This then later caused an unprotected lua error in window_set_opacity().
Signed-off-by: Uli Schlachter <psychon@znc.in>