widget: graph: NetGraph: fix unhandled exception

If Qtile trys to come up without default route, this TraceBack appears:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/qtile-0.8.0-py2.7.egg/libqtile/confreader.py", line 54, in __init__
    config = __import__(os.path.basename(fname)[:-3])
  File "/home/fsc/.config/qtile/config.py", line 139, in <module>
    widget.NetGraph(**widget_graph_defaults),
  File "/usr/local/lib/python2.7/dist-packages/qtile-0.8.0-py2.7.egg/libqtile/widget/graph.py", line 285, in __init__
    self.interface = self.get_main_iface()
  File "/usr/local/lib/python2.7/dist-packages/qtile-0.8.0-py2.7.egg/libqtile/widget/graph.py", line 321, in get_main_iface
    routes[0]
IndexError: list index out of range

Signed-off-by: Florian Scherf <fscherf@gmx.net>