Runtime fix for the mop

The following runtime has occured 40 time(s).
runtime error: Cannot execute null.clean blood().
proc name: clean (/obj/item/weapon/mop/proc/clean)
  source file: mop.dm,22
  usr: Xavier Paynter (/mob/living/carbon/human)
  src: the mop (/obj/item/weapon/mop)

Cause: Someone would mop the same vomit/whatever more than once. The first clean would qdel() the vomit, which resulted in the vomit's loc being null. The subsequent moppings would continue and eventually try to call vomit.loc.clean_blood().