With that change, if get_networks() is called without fields set to
something, _filter_nets_l3 fails because it tries to call "filters.get()"
where filters is None.
This fixes bug #1060047
Change-Id: Ia0787e7a278a3562af5409861762f067d71f2cf6
Signed-off-by: Julien Danjou <julien@danjou.info>