Fix concatenation bug in filesystem::path

Fix concatenation bug in filesystem::path

When erasing a trailing empty filename component, the output iterator
was not decremented, causing the next component to be created at the
wrong position.

	* src/filesystem/std-path.cc (path::operator+=(const path&)): Fix
	incorrect treatment of empty filename after trailing slash.
	* testsuite/27_io/filesystem/path/concat/path.cc: Test problem case.

From-SVN: r267574