Instagib Riff can cause script errors

[catornot]

essentially, it is possible for InstagibUpdateWeapons to get a null entity
(got this with bots)

callstack:

[SCRIPT SV] [info] SCRIPT ERROR: [SERVER] Entity is null
[SCRIPT SV] [info]  -> player.TakeWeaponNow( player.GetOffhandWeapon( OFFHAND_RIGHT ).GetWeaponClassName() )
[SCRIPT SV] [info] 
CALLSTACK
*FUNCTION [InstagibUpdateWeapons()] gamemodes/sh_riff_instagib.gnut line [52]
*FUNCTION [InstagibCycleWeaponsForKill()] gamemodes/sh_riff_instagib.gnut line [65]
*FUNCTION [PlayerOrNPCKilled()] mp/_base_gametype.gnut line [745]
*FUNCTION [CodeCallback_OnPlayerKilled()] mp/_base_gametype_mp.gnut line [320]
*FUNCTION [TakeDamage()] NATIVE line [-1]
*FUNCTION [HumanMeleeAttack_DoImpact()] melee/sh_melee_human.gnut line [209]
*FUNCTION [HumanMeleeAttack()] melee/sh_melee_human.gnut line [158]
*FUNCTION [CodeCallback_OnMeleeAttackAnimEvent()] melee/sh_melee.gnut line [1007]