(lack of) va_copy, and segfaults on x86_64

[Wim Lewis]

Compiling ncurses-5.7 on macosx-x86_64, I find that it segfaults 
frequently within _nc_printf_string() in printw(). The reason seems to be 
the way that the va_list is used twice (once to compute the buffer's 
length, and once to actually format the string). Like some other 
architectures with a register passing convention, va_list on this machine 
is a (pointer to a) structure, not a simple pointer to stack memory as it 
is on an i386 or vax, and so the implicit copy made by a function call 
doesn't actually keep the caller's argument pointer from being modified by 
the callee.

The solution is pretty simple, to use va_copy() to explicitly make a copy 
of the argument pointer. It looks like an (unrelated?) use of va_copy was 
added and then reverted in 20061202/20061217, though, so I'm not sure what 
issue needs to be dealt with in order to use va_copy(). (Perhaps just an 
autoconf test for its existence?)

I'd guess that this problem affects other architectures too --- I've had 
to make similar changes in the past to get stuff to run on linux-ppc, eg.

This is the change I made to my local copy --- not intended as a patch 
submission, just to show what I'm talking about. With this change 
ncurses seems to work fine:

  _nc_printf_string(const char *fmt, va_list ap)
  {
      char *result = 0;

      if (fmt != 0) {
  #if USE_SAFE_SPRINTF
!       va_list ap_copy;
!       va_copy(ap_copy, ap);
!       int len = _nc_printf_length(fmt, ap_copy);
!       va_end(ap_copy);

        if ((int) my_length < len + 1) {
            my_length = 2 * (len + 1);