northd.c: Validate port type to avoid unexpected behavior.

In ovn_igmp_group_get_ports(), it accesses a union member that should
exist only if the port is a LSP: port->peer->od->mcast_info.rtr.relay.
But in theory it is possible that the "port" is in fact a LRP, because
it is a result of ovn_port_find() with the port name coming from a SB DB
entry. So it is better to validate that first.

Signed-off-by: Han Zhou <hzhou@ovn.org>
Acked-by: Dumitru Ceara <dceara@redhat.com>
(cherry picked from commit e71873b)

northd/northd.c

@@ -4570,7 +4570,7 @@ ovn_igmp_group_get_ports(const struct sbrec_igmp_group *sb_igmp_group,
         struct ovn_port *port =
             ovn_port_find(ovn_ports, sb_igmp_group->ports[i]->logical_port);
 
-        if (!port) {
+        if (!port || !port->nbsp) {
             continue;
         }