Do not fade-in scrollbars if scrollview node has been deleted

Hi,

I experimented a JS bug : 
On webapp, I change the content of "mainframe" dynamically with Ajax, scrollviews are created on the content (not in the global strucutre ).The problem was, when the user navigate to modules very fast (and change the mainframe content, very fast), I had a Javascript error (can't call _transition on null)

So I dived a little, into the scrollbar plugin, and I found that the system call the show() function (with the flash() )with a Y.later call. The problem is that verticalNode.node (axis Y in my case) can be null, and the transition is call on null object.

That's why, i added a verticalNode.node and horizontalNode._node test, to avoid calling transition on a null object.

Hope, I have made it clear

Julien