netbsd-tests t_regex_att: Use __nonstring for a non-string constant

[jhb]

Summary

Fixes the following warning from GCC 15:

contrib/netbsd-tests/lib/libc/regex/t_regex_att.c:54:30: error: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (4 chars into 3 available) [-Werror=unterminated-string-initialization]

54 | static const char delim[3] = "\\\\\0";
   |                              ^~~~~~~~

[dim]

LGTM. Weird that I didn't see this one with clang 21, which also has the same warnings about __nonstring. But maybe I just didn't run this test?

[ngie]

Definitely could be contributed upstream.

[jhb]

Yeah, I have no idea why clang didn't warn about this one. I wonder if clang doesn't warn if the string ends with an explicit nul?