bug#18680: [PATCH] Check if argv[0] is NULL in scm_boot_guile to avoid segfault

[Nala Ginrut]

This patch is for master branch, but the bug also appears in the
current stable-2.0 I think.

With the recent patch from Eli Zaretskii, there'd be one more step for argv[0]:
scm_i_mirror_backslashes (argv[0]);

But this cause my program segfault, since I have such line:
scm_boot_guile(0, {NULL}, &guilemain, NULL);

According to C11, it's allowed[1].
argv[argc] should be NULL.
If the value of argc is greater than zero, the string pointed to by argv[0]
represents the program name. But if argc is zero, argv[0] will be NULL, so
we have to check it first to avoid segfault.

Attached patch fixed this.

[1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

0001-Check-argv-0-if-it-s-NULL-in-scm_boot_guile.patch
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