bug#7016: Bug in Speedbar

[Chong Yidong]

> The easiest way to reproduce the bug is as follows:
> 1. Load emacs with "emacs -Q"
> 2. Open the file with the above code.
> 3. semantic-mode
> 4. global-linum-mode
> 5. Open speedbar
> 6. Go to 'quick buffers' display in speedbar
> 7. Press space on the .cpp file opened and then press space twice on
> the Variables section.

Thank you for the concise recipe.  I've committed a fix, which will be
included in Emacs 23.3.