Core/Unit: Clear threat list on units when they die (fix players having aggro on corpses)

Original suggestion by glorandwarf@yahoo.com

Fixes issue #4667

--HG--
branch : trunk

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

@@ -12518,7 +12518,7 @@ bool Unit::CanHaveThreatList() const
         return false;
 
     // only alive units can have threat list
-    if (!isAlive())
+    if (!isAlive() || isDying())
         return false;
 
     // totems can not have threat list