bpo-41524: fix pointer bug in PyOS_mystr{n}icmp (GH-21845)

* bpo-41524: fix pointer bug in PyOS_mystr{n}icmp

The existing implementations of PyOS_mystrnicmp and PyOS_mystricmp
can increment pointers beyond the end of a string.

This commit fixes those cases by moving the mutation out of the condition.

* 📜🤖 Added by blurb_it.

* Address comments

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>