Fix regexp matching with atomic strings and optimised backtracking

Fix regexp matching with atomic strings and optimised backtracking

This bug occurs when an atomic pattern is matched at the end of
a string and the on-failure-keep-string-jump optimisation is
in effect, as in:

  (string-match "\\'\\(?:ab\\)*\\'" "a")

which succeeded but clearly should not (bug#58726).

Reported by Michael Heerdegen.

* src/regex-emacs.c (PREFETCH): Add reset parameter.
(re_match_2_internal): Use it for proper atomic pattern treatment.
* test/src/regex-emacs-tests.el (regexp-atomic-failure): New test.