libstdc++: Fix std::match_results::end() for failed matches [PR102667]

libstdc++: Fix std::match_results::end() for failed matches [PR102667]

The end() function needs to consider whether the underlying vector is
empty, not whether the match_results object is empty. That's because the
underlying vector will always contain at least three elements for a
match_results object that is "ready". It contains three extra elements
which are stored in the vector but are not considered part of sequence,
and so should not be part of the [begin(),end()) range.

libstdc++-v3/ChangeLog:

	PR libstdc++/102667
	* include/bits/regex.h (match_result::empty()): Optimize by
	calling the base function directly.
	(match_results::end()): Check _Base_type::empty() not empty().
	* testsuite/28_regex/match_results/102667.C: New test.