triggercontrolsource: add missing end_iter check for sequence
triggercontrolsource: add missing end_iter check for sequence

Avoid accessing the end-iter, this is a marker without a data field.

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