Core/AI: prevent launching a new MoveChase if victim didn't change

Thanks ccrs for feedback :P

(cherry picked from commit b6b0353bff)

src/server/game/AI/CreatureAI.cpp

@@ -251,7 +251,7 @@ bool CreatureAI::UpdateVictim()
     if (!me->HasReactState(REACT_PASSIVE))
     {
         if (Unit* victim = me->SelectVictim())
-            if (!me->IsFocusing(nullptr, true))
+            if (!me->IsFocusing(nullptr, true) && victim != me->GetVictim())
                 AttackStart(victim);
 
         return me->GetVictim() != nullptr;