bug#18473: 24.4.50; SEGFAULT when vconcatting more than 2048 vectors

[Stefan Monnier]

> -       set_backtrace_args (specpdl_ptr - 1, vals);
> -       set_backtrace_nargs (specpdl_ptr - 1, XINT (numargs));
> +       set_backtrace_args (backtrace_top (), vals, XINT (numargs));

I'm not thrilled at the idea of having to call backtrace_top every time,
instead of just using "specpdl_ptr - 1".  I'd much rather compute
specpdl_ptr - 1 before calling USE_SAFE_ALLOCA, for example.

BTW, don't we have the same problem for the subsequent call to
backtrace_debug_on_exit (and this one is worse because it is shared
among all the different kinds of calls, so we *really* don't want to
slow it down).


        Stefan