basesrc: Only set duration/position query values in case of query success
basesrc: Only set duration/position query values in case of query success

Currently, the query values are being set even if the query itself was
determined to have failed. Fix this to ensure the values are only set in
case of a query success.

https://bugzilla.gnome.org/show_bug.cgi?id=760479

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