[STUMP] Crash in VLC "Open File" dialog

[Adlai Chandrasekhar]

Backtrace at http://paste.lisp.org/display/95662

I had VLC open, and then went to the Open File dialog. Then, while the dialog 
was still open, I hit C-t C-t to switch windows, and poof.

Some thoughts that simias and I had in #stumpwm:

 - Things seem doomed at (7F5B5001EA10) : 11 (TRANSIENTS-OF NIL) 69

 - #'shadows-of relies on #'window-by-id to return something that's a valid 
argument to #'transients-of; however, #'window-by-id fails silently when given 
nil as an argument.

I'm not familiar with this corner of the X protocol (nor with any other 
corner), but the silent failure in #'window-by-id seems odd. Any suggestions?

Adlai