Fixes multiple runtimes in mutiny mode.

Example runtime:

runtime error: Cannot read null.assigned_role
proc name: get crew to terminate (/datum/directive/terminations/financial_crisis/get_crew_to_terminate)
  source file: financial_crisis_directive.dm,11
  usr: null
  src: /datum/directive/terminations/... (/datum/directive/terminations/financial_crisis)
  call stack:
/datum/directive/terminations/... (/datum/directive/terminations/financial_crisis): get crew to terminate()
/datum/directive/terminations/... (/datum/directive/terminations/financial_crisis): meets prerequisites()
mutiny (/datum/game_mode/mutiny): get directive candidates()
mutiny (/datum/game_mode/mutiny): pre setup()
/datum/controller/gameticker (/datum/controller/gameticker): setup()
/datum/controller/gameticker (/datum/controller/gameticker): pregame()
/datum/controller/game_control... (/datum/controller/game_controller): setup()
Merge pull request #5366 from alex-gh/mutiny_runtime_fix

Fixes multiple runtimes in mutiny mode.