Fix 'The Undying' and 'The Immortal' achievements.

src/server/game/Entities/Unit/Unit.cpp

@@ -12750,6 +12750,10 @@ void Unit::setDeathState(DeathState s)
         // do not why since in IncreaseMaxHealth currenthealth is checked
         SetHealth(0);
         SetPower(getPowerType(), 0);
+
+        // players in instance don't have ZoneScript, but they have InstanceScript
+        if (ZoneScript* zoneScript = GetZoneScript() ? GetZoneScript() : (ZoneScript*)GetInstanceScript())
+            zoneScript->OnUnitDeath(this);
     }
     else if (s == JUST_ALIVED)
         RemoveFlag (UNIT_FIELD_FLAGS, UNIT_FLAG_SKINNABLE); // clear skinnable for creature and player (at battleground)