illumos gate

After #13727, when using the undocumented DEMANGLE_DEBUG environment variable to verify some demangling cases, I tripped the following assert:

assertion failed for thread 0xfef92a40, thread-id 1: !name_empty(n), file ../common/cxx_util.c, line 156

This turns out to be somewhat trivial to recreate -- simply run DEMANGLE_DEBUG=1 demangle '_Z'.

The relevant stack is:

 feec21e3 syscall  (803b590, 320, f0d960a8, fef20000, 803b5f5, fef20000) + 13
 feea0e2d __set_panicstr (803b590)
 feea1dd0 _assfail (fef5ed9e, fef5ed79, 9c) + 1a0
 feea1ec7 assfail3 (fef5ed9e, fef5ed79, 9c, fef40800, fef60078, fedf2b38)
 fef540fd name_at  (803ba0c, 0) + 99
 fef5436b name_top (803ba0c) + 1c
 fef5c766 print_name (803ba0c, 8062310) + 1f
 fef5cb80 dump     (803b9e0, 8062310) + 18

From that, the cause is fairly clear:

static void
print_name(const name_t *n, FILE *out)
{
        const str_pair_t *sp = name_top((name_t *)n);
        size_t i;

        (void) fprintf(out, "Name:\n");

        if (name_len(n) == 0)
                return;
...

We shouldn't assign sp until we've verified that n isn't empty.