Fixes an AI Runtime.

runtime error: Cannot read null.layer
proc name: Life (/mob/living/silicon/ai/Life)
  source file: life.dm,54
  usr: null
  src: AM (/mob/living/silicon/ai)
  call stack:
AM (/mob/living/silicon/ai): Life()
/datum/controller/game_control... (/datum/controller/game_controller): process mobs()
/datum/controller/game_control... (/datum/controller/game_controller): process()

Thanks to SoS for the Runtime logs.

As you can see, the cause of this is that the coder who wrote AI life() (Or added on the blindness part) Named a var "blind" which is already the name of the mob screen object, they probably though they'd sanity checked the screen object, because of seeing their Identically named var "blind"