test_overflow: fix an IS_ERR() vs NULL bug

root_device_register() returns error pointers, it never returns NULL.

Fixes: ca90800a91ba ("test_overflow: Add memory allocation overflow tests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
1 file changed