sink: Only add processing latency if upstream is live
sink: Only add processing latency if upstream is live

Only add it if upstream is live, otherwise leave the latency at 0.

https://bugzilla.gnome.org/show_bug.cgi?id=640610

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