pipeline: g-i: allow clock to be NULL in gst_pipeline_use_clock()
pipeline: g-i: allow clock to be NULL in gst_pipeline_use_clock()

https://bugzilla.gnome.org/show_bug.cgi?id=705751

mparis/gstreamer,lovebug356/gstreamer,cfoch/gstreamer,Lachann/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,mparis/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,cfoch/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,centricular/gstreamer,cablelabs/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,krieger-od/gstreamer,mparis/gstreamer,Lachann/gstreamer,justinjoy/gstreamer,centricular/gstreamer,mparis/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,centricular/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,lubosz/gstreamer,krichter722/gstreamer,cfoch/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,centricular/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,krichter722/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,lubosz/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,ensonic/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,mparis/gstreamer