Catch gs_listener_dbus_init failures

In cases where the dbus connection fails, gs_listener_dbus_init
will fail and return FALSE. Previously, this status code was not
checked in the gs_listener constructor, resulting in a silent failure
that eventually resulted in a Segmentation Fault when the listener's
system_connection pointer gets used.

Add a g_assert to ensure gs_listener_dbus_init succeeds, or bail out
gracefully on failures.