Fix a crash in generic_vehicleAI_toc5AI.

gdb log:

#3  0x0000000000ce25c9 in Vehicle::GetPassenger (this=0x0, seatId=0 '\0')
#4  0x0000000000f10820 in generic_vehicleAI_toc5AI::UpdateAI (
    this=0x7f1c123c3000, uiDiff=118)

--HG--
branch : trunk

src/scripts/northrend/crusaders_coliseum/trial_of_the_champion/boss_grand_champions.cpp

@@ -261,6 +261,12 @@ struct generic_vehicleAI_toc5AI : public npc_escortAI
         //dosen't work at all
         if (uiShieldBreakerTimer <= uiDiff)
         {
+            if (!pVehicle)
+            {
+                pVehicle = m_creature->GetVehicleKit();
+                return;
+            }
+
             if (Unit* pPassenger = pVehicle->GetPassenger(SEAT_ID_0))
             {
                 Map::PlayerList const& players = m_creature->GetMap()->GetPlayers();