tools: iio: iio_utils: fix digit calculation

tools: iio: iio_utils: fix digit calculation

The iio_utils uses a digit calculation in order to know length of the
file name containing a buffer number. The digit calculation does not
work for number 0.

This leads to allocation of one character too small buffer for the
file-name when file name contains value '0'. (Eg. buffer0).

Fix digit calculation by returning one digit to be present for number
'0'.

Fixes: 096f9b862e60 ("tools:iio:iio_utils: implement digit calculation")
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/Y0f+tKCz+ZAIoroQ@dc75zzyyyyyyyyyyyyycy-3.rev.dnainternet.fi
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>