[racket] match and debug in Advanced Student

This problem was caused by a bug in the bytecode compiler. The bug was
exposed by usual code (including a `set!' to an other unused procedure
`let' binding) in the "Debug" button's transformation of the expanded
`match' form.

I've pushed a repair to the git repo.

At Tue, 08 Nov 2011 23:17:32 -0500, Prabhakar Ragde wrote:
> I never use the Debug button, but I let my students start using Advanced 
> Student today, and of course they tried it out.
> 
> The following program induces a strange error when the Debug button is 
> pressed.
> 
> (match 3
>    [`(,x) 3]
>    [x 3])
> 
> The error is:
> internal error: skipped binding is used
> 
> after which the program hangs, though it can be stopped with the Stop 
> button. --PR
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