[rilmodem] Fix assert warning in development mode

Before configuration is stored LTE configuration storage is empty
and causes assert notification in devel mode when keyfile is read.
This fix removes the problem.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #140 from jkangas/next

Request for pulling assert warning fix to next branch
[voicecall] Fix compiler warnings

Fix 'unused variable' compiler warnings introduced by commit 6aafbe93744b366518fe5cd69661d5998ae70958.

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>