Fixes issue when using SafeRemoveEntity/SafeRemoveEntityDelayed giving error on "[ERROR] lua/uncludes/util.lua:184/195: attempt to call method 'IsValid' (a nil value)" under certain circumstances when an entity is removed ( either several times using SafeRemoveEntity / Remove combo, or when trying to remove a swep that has been removed / was in the process of being removed or when trying to remove a swep that was just holstered ).
Per Robot, altered "Replace the whole
!ent || ( ent.IsValid && !ent:IsValid() )
with
!IsValid( ent )"

Additionally, SafeRemoveEntityDelayed calls SafeRemoveEntity so ent:IsPlayer( ) isn't needed, but to prevent an extra timer from being created for an entity we won't remove anyway, I added that check.
Merge pull request #769 from Acecool/fix_util_saferemoveentity_errors

Fixes error in SafeRemoveEntity/SafeRemoveEntityDelayed under certain circumstances