[17+] check after second call to pg_strnxfrm is too strict, relax it

I noticed this comment in selfuncs.c regarding strxfrm():

/*
* Some systems (e.g., glibc) can return a smaller value from the
* second call than the first; thus the Assert must be <= not ==.
*/

Some callers of pg_strnxfrm() are not allowing for that possibility.

Patch attached, which should be backported to 17.

Regards,
Jeff Davis