fix(SmartAI): Ensure immediate combat engagement (#21854)

NPCs will stop their current waypoint and start combat.

src/server/game/AI/SmartScripts/SmartAI.cpp

@@ -848,6 +848,7 @@ void SmartAI::AttackStart(Unit* who)
             {
                 me->GetMotionMaster()->MovementExpired();
                 me->StopMoving();
+                me->GetMotionMaster()->Clear(false);
             }
 
             me->GetMotionMaster()->MoveChase(who);