[plt-scheme] SIGSEGV fault on (nil)

Hello,

I ran into a crash in a particular combination of delimited continuations
and sandbox.  I tried many ways to make it fail for simpler, non-interactive
cases but they all made the bug disappear, so the reproduction is a bit
cumbersome:

 darcs get http://zwizwa.be/darcs/snot
 cd snot/snot
 mzscheme start.ss

 Then paste 3 expressions to stdin, follow the instructions
 in crash.script

I tried this on amd64/i386 with SVN 15520 from 20090721, and
on amd64 with today's SVN 15895.

Cheers,
Tom