misc: Fix rare fortify crash on wchar funcs. [BZ 29030]

If `__glibc_objsize (__o) == (size_t) -1` (i.e. `__o` is unknown size), fortify
checks should pass, and `__whatever_alias` should be called.

Previously, `__glibc_objsize (__o) == (size_t) -1` was explicitly checked, but
on commit a643f60c, this was moved into `__glibc_safe_or_unknown_len`.

A comment says the -1 case should work as: "The -1 check is redundant because
since it implies that __glibc_safe_len_cond is true.". But this fails when:
* `__s > 1`
* `__osz == -1` (i.e. unknown size at compile time)
* `__l` is big enough
* `__l * __s <= __osz` can be folded to a constant
(I only found this to be true for `mbsrtowcs` and other functions in wchar2.h)

In this case `__l * __s <= __osz` is false, and `__whatever_chk_warn` will be
called by `__glibc_fortify` or `__glibc_fortify_n` and crash the program.

This commit adds the explicit `__osz == -1` check again.
moc crashes on startup due to this, see: https://bugs.archlinux.org/task/74041



Minimal test case (test.c):
    #include <wchar.h>

    int main (void)
    {
        const char *hw = "HelloWorld";
        mbsrtowcs (NULL, &hw, (size_t)-1, NULL);
        return 0;
    }

Build with:
    gcc -O2 -Wp,-D_FORTIFY_SOURCE=2 test.c -o test && ./test

Output:
    *** buffer overflow detected ***: terminated

Fixes: BZ #29030
Signed-off-by: Joan Bruguera <joanbrugueram@gmail.com>
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 33e03f9c)