Runtime fixes.

runtime error: Cannot execute null.on reagent change().
proc name: del reagent (/datum/reagents/proc/del_reagent)
  source file: Chemistry-Holder.dm,308
  usr: null
  src: /datum/reagents (/datum/reagents)
  call stack:
/datum/reagents (/datum/reagents): del reagent("pacid")
/datum/reagents (/datum/reagents): clear reagents()
Chemsmoke (/datum/chemical_reaction/chemsmoke): on reaction(/datum/reagents (/datum/reagents), null)

runtime error: Division by zero
proc name: Topic (/datum/song/Topic)
  source file: musician.dm,193
  usr: Tico-Tico Tambien (/mob/living/carbon/human)
  src: Untitled (/datum/song)
  call stack:
Untitled (/datum/song): Topic("src=\[0x210054cd];import=1", /list (/list))
Merge pull request #1053 from Giacomand/musician_fix

 Runtime fixes.