gstclock: Fix GST_STIME_ARGS
gstclock: Fix GST_STIME_ARGS

It wasn't properly handling GST_CLOCK_STIME_NONE and always use the
sign marker (+/-) to make it easier to identify signed values in
logs

https://bugzilla.gnome.org/show_bug.cgi?id=758870

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