fixed issue with de-normalized quaternion, causing acosf to fail

See http://www.bulletphysics.com/Bullet/phpBB3/viewtopic.php?f=9&t=1440
Thanks Proctoid for reporting.

src/LinearMath/btTransformUtil.h

@@ -121,6 +121,10 @@ public:
 		dmat.getRotation(dorn);
 #endif//USE_QUATERNION_DIFF
 	
+		///floating point inaccuracy can lead to w component > 1..., which breaks 
+
+		dorn.normalize();
+		
 		angle = dorn.getAngle();
 		axis = btVector3(dorn.x(),dorn.y(),dorn.z());
 		axis[3] = btScalar(0.);