Fix: directory-handle: use of free'd handle on fstat() error

On an error to fstat a directory handle, a directory handle is
released and is still initialized as if the error had not occurred.

Return NULL early from lttng_directory_handle_create_from_dirfd()
to prevent those erroneous accesses.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia2a3bb1cfe8c90d3f7f87a68286a9b8524694d3c