selftests: Uninitialized variable in test_cgcore_proc_migration()

The "c_threads" variable is used in the error handling code before it
has been initialized

Fixes: 11318989 ("selftests: cgroup: Add task migration tests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>