timedvaluecontrolsource: Check that the only iter is the end iter in the GSequence
timedvaluecontrolsource: Check that the only iter is the end iter in the GSequence

Previous patch was assuming that if the returned iter was the last iter
the GSequence was empty, which is obviously wrong.

centricular/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,krichter722/gstreamer