libstdc++: Use non-throwing increment in recursive_directory_iterator [PR 97731]

libstdc++: Use non-throwing increment in recursive_directory_iterator [PR 97731]

As described in the PR, the recursive_directory_iterator constructor
calls advance(ec), but ec is a pointer so it calls _Dir::advance(bool).
The intention was to either call advance() or advance(*ec) depending
whether the pointer is null or not.

This fixes the bug and renames the parameter to ecptr to make similar
mistakes less likely in future.

libstdc++-v3/ChangeLog:

	PR libstdc++/97731
	* src/filesystem/dir.cc (recursive_directory_iterator): Call the
	right overload of _Dir::advance.
	* testsuite/experimental/filesystem/iterators/97731.cc: New test.