Fix: config_is_valid()
Fix: config_is_valid()

The function was receiving a GSList with struct config as input and treating as
if it were a GSList with knot_message_config. Although it was working correctly
it was corrected for better understanding and avoid mistakes in the future.

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source