avahi-daemon segfaults when a txt-record has no value

[小畑 勝也]

This bug is reported upstream( https://github.com/lathiat/avahi/issues/247 ) but ubuntu focal did not fix it.

Version: avahi-daemon (0.7-4ubuntu7.1) on Focal Fossa

Service XML causing the segfault:

```
<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name replace-wildcards="yes">Printer</name>
  <service protocol="ipv4">
    <type>_ipp._tcp.</type>
    <host-name>printer.local</host-name>
    <port>631</port>
    <txt-record>note=</txt-record>
  </service>
</service-group>
```

The line `<txt-record>note=</txt-record>` causes avahi to segfault.

The Workaround I found was: `<txt-record>note=""</txt-record>` I.e. setting the `note` to an empty string.

It can fix to merge this ( https://github.com/lathiat/avahi/pull/226 ).

This bug fixed on upstream version 0.8, so this bug remains on bionic and focal.

[Paul White (paulw2u)]

'focal' is a release name and not the name of a
package currently in a supported release of Ubuntu.