* libstdc++-v3/include/bits/regex.h (regex_iterator::regex_iterator()):
	Define end() as _M_pregex == nullptr.
	* libstdc++-v3/include/bits/regex.tcc (regex_iterator::operator==(),
	regex_iterator::operator++()): Fix operator==() and operator++() to
	look at null-ness of _M_pregex on both sides.
	* testsuite/28_regex/regression.cc: New testcase.

From-SVN: r242025