ceph: silence a checker warning in mdsc_show()

The problem is that if ceph_mdsc_build_path() fails then we set "path"
to NULL and the "pathlen" variable is uninitialized.  Then we call
ceph_mdsc_free_path(path, pathlen) to clean up.  Since "path" is NULL,
the function is a no-op but Smatch and UBSan still complain that
"pathlen" is uninitialized.

This patch doesn't change run time, it just silence the warnings.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>