Prevent nil error which occures sometimes at end of M+ run

[doadin]

796x RaiderIO/core.lua:7943: attempt to perform arithmetic on field 'killed' (a nil value) [string "@RaiderIO/core.lua"]:7943: in function Update' [string "@RaiderIO/core.lua"]:7927: in function Setup' [string "@RaiderIO/core.lua"]:9922: in function SetUIBosses' [string "@RaiderIO/core.lua"]:9616: in function OnBossKill' [string "@RaiderIO/core.lua"]:8486: in function GetSummary' [string "@RaiderIO/core.lua"]:9671: in function UpdateShown' [string "@RaiderIO/core.lua"]:10199: in function callbackFunc' [string "@RaiderIO/core.lua"]:1126: in function SendEvent' [string "@RaiderIO/core.lua"]:1061: in function <RaiderIO/core.lua:1057>

Locals: Skipped (In Encounter)

prevent the nil error by verifying the value exists.