Fixes runtime on returnToPool being given a null argument.
Created by: gbasood
istype checks for null (sometimes), but in the case of an argument like null.loc, it doesn't. This causes a runtime error when returnToPool is given a null argument.
Code I used to test this in an empty project:
client/New()
..()
var/mob/test = new
del(test)
world << istype(test.loc, /mob) //Runtime here