gh-132915: Fix compiler warning in fcntlmodule.c

[vstinner]

Fix the warning:

Modules/fcntlmodule.c:27:36: warning: initializer-string for
array of 'char' truncates NUL terminator but destination lacks
'nonstring' attribute (9 chars into 8 available)
[-Wunterminated-string-initialization]
static const char guard[GUARDSZ] = "\x00\xfa\x69\xc4\x67\xa3\x6c\x58";

• Issue: Detect buffer overflow in fcntl.fcntl() and fcntl.ioctl() #132915

[vstinner]

It seems to be a new warning of GCC 15 (my GCC version on Fedora 42).

destination lacks 'nonstring' attribute

I prefer to add an unused null byte at the end instead of adding a new macro for this new attribute:
https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute

[vstinner]

cc @serhiy-storchaka

[vstinner]

Oh, another function needs the attribute. So I decided to add the attribute instead :-(

• GCC 15 warnings: destination lacks 'nonstring' attribute #133256
• gh-133256: Add _Py_NONSTRING macro #133257

[serhiy-storchaka]

We could just use array initializer: {0, 250, 105, 196, 103, 163, 108, 88}.