iio: test: Add check against NULL for buffer in tests.

Add KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buf) for every test.
Also use ARRAY_SIZE(values) where it is possible.

Signed-off-by: Andriy Tryshnivskyy <andriy.tryshnivskyy@opensynergy.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20211105100501.1904-2-andriy.tryshnivskyy@opensynergy.com


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