Solves a runtime when someone tries to mop up blood.

proc name: afterattack (/obj/item/weapon/mop/afterattack)
  source file: mop.dm,45
runtime error: undefined proc or verb /obj/effect/decal/cleanable/blood/tracks/footprints/clean().
Merge pull request #7109 from Ccomp5950/mop_fix

Solves a runtime when someone tries to mop up blood.