init script doesn't work correctly with multiple IPs on the external interface

[Nathan Rennie-Waldock]

Parts of the init script fail if the external interface has multiple IPs assigned.

systemctl status output:

   CGroup: /system.slice/miniupnpd.service
           └─29951 /usr/sbin/miniupnpd -i eth0 -o 5.xxx.xxx.xxx 51.xxx.xxx.xxx 51.xxx.xxx.xxx -a tap1 -N -f /etc/miniupnpd/miniupnpd.conf

Aug 05 12:46:35 miniupnpd[29926]: Bad argument `51.xxx.xxx.xxx'
Aug 05 12:46:35 miniupnpd[29926]: Try `iptables -h' or 'iptables --help' for more information.
Aug 05 12:46:35 miniupnpd[29926]: * Starting UPnP devices daemon MiniUPnPd
Aug 05 12:46:35 miniupnpd[29926]: Unknown option: 51.xxx.xxx.xxx
Aug 05 12:46:35 miniupnpd[29926]: Unknown option: 51.xxx.xxx.xxx
Aug 05 12:46:35 miniupnpd[29951]: HTTP listening on port 42522
Aug 05 12:46:35 miniupnpd[29951]: HTTP IPv6 address given to control points : [2001:41d0:8:xxxx::]
Aug 05 12:46:35 miniupnpd[29951]: Listening for NAT-PMP/PCP traffic on port 5351
Aug 05 12:46:35 systemd[1]: Started LSB: daemon providing UPnP Internet Gateway Device (IGD) services.
Aug 05 12:46:35 miniupnpd[29926]: ...done.

The problem is this line:
EXTIP="$(LC_ALL=C ${IP} addr show ${MiniUPnPd_EXTERNAL_INTERFACE} | grep "inet " | awk '{ print $2 }' | cut -d"/" -f1)"
It should be changed to (preferably with another variable in /etc/default/miniupnpd to override it):
EXTIP="$(LC_ALL=C ${IP} addr show ${MiniUPnPd_EXTERNAL_INTERFACE} | grep -m1 "inet " | awk '{ print $2 }' | cut -d"/" -f1)"

Or, according to the manpage it could be removed (to use the default address) and leave a variable to override it.
       -o address
              address used to connect to the internet. default address of the interface will be used if not specified.