Scripts/EyeOfEternity: Fix crash

Fix crash happening when a player would kill a Melee npc on the disks during Malygos fight and mount it before it ended the waypoint path.
Close #15900

(cherry picked from commit 6e296ddaa2)

src/server/scripts/Northrend/Nexus/EyeOfEternity/boss_malygos.cpp

@@ -1244,8 +1244,9 @@ public:
                 ++_wpCount;
             }
             else if (Vehicle* hoverDisk = me->GetVehicleKit())
-                if (Unit* lordPassenger = hoverDisk->GetPassenger(0))
-                    lordPassenger->ToCreature()->AI()->DoAction(ACTION_SET_DISK_VICTIM_CHASE);
+                if (Unit* passenger = hoverDisk->GetPassenger(0))
+                    if (Creature* lordPassenger = passenger->ToCreature())
+                        lordPassenger->AI()->DoAction(ACTION_SET_DISK_VICTIM_CHASE);
         }
 
     private: