kselftest/cgroup: fix variable dereferenced before check warning

cg_name(const char *root, const char *name) is always called with
non-empty root and name arguments, so there is no sense in checking
it in the function body (after using in strlen()).

Signed-off-by: Roman Gushchin <guro@fb.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>