avahi-discover SEGVs if no X display

[Chad Miller]

$ DISPLAY=0 avahi-discover
/usr/lib/python2.7/dist-packages/gtk-2.0/gtk/__init__.py:57: GtkWarning: could not open display
  warnings.warn(str(e), _gtk.Warning)
/usr/bin/avahi-discover:64: Warning: invalid (NULL) pointer instance
  self.ui.add_from_file(path)
/usr/bin/avahi-discover:64: Warning: g_signal_connect_data: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed
  self.ui.add_from_file(path)
...
  self.ui.add_from_file(path)
Segmentation fault (core dumped)

Probably a null deref, but still shouldn't happen if a program is checking its return codes for errors.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: avahi-discover 0.6.32~rc+dfsg-1ubuntu2
Uname: Linux 4.5.0-040500-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: GNOME
Date: Mon May 23 09:24:01 2016
EcryptfsInUse: Yes
PackageArchitecture: all
SourcePackage: avahi
UpgradeStatus: No upgrade log present (probably fresh install)