[CM] possible bug in hook invocation

[bil at ccrma.Stanford.EDU]

This could be a GC problem, but it's a circuitous path if so.
Can you send me enough code to try to recreate the segfault?
On the other question, a lambda form is only deemed safe
if its body is safe.  If you hit the segfault in gdb,
a stacktrace would be helpful.  To see if the GC is
the problem, build s7 with -g3 and S7_DEBUGGING=1, then
when you crash in safe_closure_p_a_1

p sc->code[0]

It should be a "closure" in s7 terminology (type==39 in the gdb
output).  If it's free (type==0), go to the end of the gdb
printout, and you'll see where the GC freed it and so on.
(Actually S7_DEBUGGING will tell you most of this before
falling into gdb).