[3.11] gh-105235: Prevent reading outside buffer during mmap.find() (… (#106710)

[3.11] gh-105235: Prevent reading outside buffer during mmap.find() (GH-105252)

* Add a special case for s[-m:] == p in _PyBytes_Find

* Add tests for _PyBytes_Find

* Make sure that start <= end in mmap.find.
(cherry picked from commit ab86426a3472ab68747815299d390b213793c3d1)