iio: core: Leave private pointer NULL when no private data supplied

iio: core: Leave private pointer NULL when no private data supplied

In iio_device_alloc() when size of the private data is 0,
the private pointer is calculated to point behind the valid data.
Leave it NULL when no private data supplied.

Fixes: 6d4ebd565d15 ("iio: core: wrap IIO device into an iio_dev_opaque object")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240304140650.977784-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>