iio: Be consistent with allocation result tests.

Make both allocation result tests use the same format

    if (!ptr)

Signed-off-by: Joe Simmons-Talbott <joetalbott@gmail.com>
Link: https://lore.kernel.org/r/20220718020348.32047-1-joetalbott@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>