A client process could create and destroy GClueSimple multiple times in its
lifetime. Since the service keeps the Client (and it's associated location)
object alive until the D-Bus connection from the client process remains
alive and gclue_simple_new*() awaits for a location fix on the client it
gets from service, subsequent calls to gclue_simple_new*() (after first
one) will likely not return anytime soon.

With this patch, GClueSimple now checks for any existing location object on
the client object and doesn't keep waiting for location update if one is
available already.