Fixes vis_enabled becoming unobtainable*

*(If something happens to apply plane_holder to mob before login)
Yeah a feature somewhere down there relied on the mob getting its plane_holder before client (and thus making the only vis_enabled init right here that required the lack of plane_holder on login completely unobtainable) and I was vaguely told to fix this here instead of fixing the forementioned downstream feature.

code/modules/mob/login.dm

@@ -50,6 +50,7 @@
 
 	if(!plane_holder) //Lazy
 		plane_holder = new(src) //Not a location, it takes it and saves it.
+	if(!vis_enabled)
 		vis_enabled = list()
 	client.screen += plane_holder.plane_masters
 	recalculate_vis()