re PR libstdc++/64140 (match_results.prefix() returns an incorrect result if regex_iterator holds a zero-length match)

	PR libstdc++/64140
	* include/bits/regex.tcc (regex_iterator<>::operator++): Update
	prefix.matched after modifying prefix.first.
	* testsuite/28_regex/iterators/regex_iterator/char/64140.cc: New
	testcase.

From-SVN: r218340