Fixed crash in debug mode when command line option -N is used.

src/karts/kart.cpp

@@ -2163,7 +2163,7 @@ void Kart::updateEngineSFX()
     if(isOnGround())
     {
         float max_speed = m_max_speed->getCurrentMaxSpeed();
-        assert(max_speed>0);
+
         // Engine noise is based half in total speed, half in fake gears:
         // With a sawtooth graph like /|/|/| we get 3 even spaced gears,
         // ignoring the gear settings from stk_config, but providing a

src/karts/max_speed.cpp

@@ -51,6 +51,8 @@ MaxSpeed::MaxSpeed(AbstractKart *kart)
     // Initialise m_add_engine_force since it might be queried before
     // update() is called.
     m_add_engine_force  = 0;
+    // This can be used if command line option -N is used
+    m_current_max_speed = 0;
 }   // MaxSpeed
 
 // ----------------------------------------------------------------------------