[rilmodem] check that received numeric value is valid

Existing numeric value with invalid content seems to be able to
cause dbus crash. This fix prevents the sending if the value does
not match with ril specification.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #128 from jkangas/master

Request for pulling operator name numeric value validity check to main branch