PR libstdc++/83600 fix end iterator for unready std::match_results

PR libstdc++/83600 fix end iterator for unready std::match_results

	PR libstdc++/83600
	* include/bits/regex.h (match_results::end()): Return valid iterator
	when not ready.
	* testsuite/28_regex/match_results/ctors/char/default.cc: Check that
	unready objects are empty and have equal begin and end iterators.
	* testsuite/28_regex/match_results/ctors/wchar_t/default.cc: Likewise.

From-SVN: r256014