iio: adc: stm32-adc: harden against NULL pointer deref in stm32_adc_probe()

of_match_device() may fail and returns a NULL pointer.

In practice there is no known reasonable way to trigger this, but
in case one is added in future, harden the code by adding the check

Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Link: https://lore.kernel.org/r/tencent_994DA85912C937E3B5405BA960B31ED90A08@qq.com


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