harness: Unset sink_forward_pad before tearing down sink_harness
harness: Unset sink_forward_pad before tearing down sink_harness

Set the sink_forward_pad to NULL before tearing down sink_harness to
avoid that the harness tries to forward events/queries to it while it's
tearing down.

https://bugzilla.gnome.org/show_bug.cgi?id=761904

centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer