When concatenating a path ending in a root-directory onto another path,
we added an empty filename to the end of the path twice, but only
reserved space for one. That meant the second write went past the end of
the allocated buffer.

	PR libstdc++/92853
	* src/c++17/fs_path.cc (filesystem::path::operator+=(const path&)):
	Do not process a trailing directory separator twice.
	* testsuite/27_io/filesystem/path/concat/92853.cc: New test.
	* testsuite/27_io/filesystem/path/concat/path.cc: Test more cases.

From-SVN: r279110