Fix objective runtime

When reading fromg gases you must assert_gas

The following runtime has occured 59 time(s).
runtime error: cannot read from list
proc name: check special completion (/datum/objective_item/steal/plasma/check_special_completion)
  source file: objective_items.dm,97
  usr: null
  src: 28 moles of plasma (full tank) (/datum/objective_item/steal/plasma)

code/game/gamemodes/objective_items.dm

@@ -94,7 +94,9 @@
 /datum/objective_item/steal/plasma/check_special_completion(obj/item/weapon/tank/T)
 	var/target_amount = text2num(name)
 	var/found_amount = 0
+	T.air_contents.assert_gas("plasma")
 	found_amount += T.air_contents.gases["plasma"][MOLES]
+	T.air_contents.garbage_collect()
 	return found_amount>=target_amount