kunit: Fix potential null dereference in kunit_device_driver_test()

kunit_kzalloc() may return a NULL pointer, dereferencing it without
NULL check may lead to NULL dereference.
Add a NULL check for test_state.

Link: https://lore.kernel.org/r/20241115054335.21673-1-zichenxie0106@gmail.com


Fixes: d03c720e ("kunit: Add APIs for managing devices")
Signed-off-by: Zichen Xie <zichenxie0106@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>