OsgCar.cpp: Do not create SDCarShader on "none" shaders

Even if "none" shaders were selected, the game would still parse
car.frag and execute the shaders, which is not the expected behaviour.

src/modules/graphic/osggraph/Car/OsgCar.cpp

@@ -644,6 +644,7 @@ osg::ref_ptr<osg::Node> SDCar::loadCar(tCarElt *Car, bool tracktype, bool subcat
     else
         this->reflectionMappingMethod = REFLECTIONMAPPING_OFF;
 
+    if (_carShader > 0)
         this->shader = new SDCarShader(car_shaded_body.get(), this);
 
     this->reflectionMapping = new SDReflectionMapping(this);