launcher: fix leak in launcher_weston_launch_connect()

Leak found running drm-smoke-test with ASan.

Do not forget to free the launcher before returning when we can't open
the launcher fd. Also, just set 'out = launcher' after all error paths,
otherwise we give the caller a stale pointer.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>