Fixed #2294 (crash when kart is off road and not on material).

src/modes/linear_world.cpp

@@ -179,7 +179,8 @@ void LinearWorld::update(float dt)
         // might get too close to another part of the track, shortly
         // jump to position one, then on reset fall back to last)
         if (!kart_info.getTrackSector()->isOnRoad() &&
-            kart->getMaterial()->isDriveReset())
+            (!kart->getMaterial() ||
+              kart->getMaterial()->isDriveReset())     )
             continue;
         kart_info.getTrackSector()->update(kart->getFrontXYZ());
         kart_info.m_overall_distance = kart_info.m_race_lap