Don't freely grant max speed increase on race start, first check if the bonus was obtained

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@8642 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/karts/kart.cpp

@@ -1379,7 +1379,7 @@ void Kart::updatePhysics(float dt)
         m_has_started = true;
         float f       = m_kart_properties->getStartupBoost();
         m_vehicle->activateZipper(f);
-        MaxSpeed::increaseMaxSpeed(MS_INCREASE_ZIPPER, +10,
+        MaxSpeed::increaseMaxSpeed(MS_INCREASE_ZIPPER, 0.6f*f,
                                    5.0f, 5.0f);
 
     }