Right now you can configure any listen address for the SNMP service.
If only one listen address is configured, and it is not assigned to the system, SNMP will fail to start and the user won't know, why.
User python vyos.validate.is_addr_assigned() intruduced some weeks ago (https://github.com/vyos/vyos-1x/blob/current/python/vyos/validate.py#L41-L63) and add a section to snmp.py:validate().
This glitch was reported by @masterit