clock: fix racy shutdown clock id leak
clock: fix racy shutdown clock id leak

Clock IDs were leaked if the clock got disposed before the worker thread
got a chance to reap unscheduled entries.

Fixes bug #630439.

ahmedammar/platform_external_gst_gstreamer,cablelabs/gstreamer,justinjoy/gstreamer,magcius/gstreamer,lubosz/gstreamer,magcius/gstreamer,mparis/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,drothlis/gstreamer,drothlis/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,magcius/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,Distrotech/gstreamer,ahmedammar/platform_external_gst_gstreamer,StreamUtils/gstreamer,ylatuya/gstreamer,surround-io/gstreamer,Lachann/gstreamer,lubosz/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,Distrotech/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,ahmedammar/platform_external_gst_gstreamer,mparis/gstreamer,cablelabs/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,mrchapp/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,centricular/gstreamer,lubosz/gstreamer,centricular/gstreamer,jpakkane/gstreamer,ahmedammar/platform_external_gst_gstreamer,krichter722/gstreamer,lubosz/gstreamer,Distrotech/gstreamer,mrchapp/gstreamer,krichter722/gstreamer,centricular/gstreamer,jpakkane/gstreamer,Distrotech/gstreamer,surround-io/gstreamer,drothlis/gstreamer,jpxiong/gstreamer,centricular/gstreamer,Lachann/gstreamer,collects/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,collects/gstreamer,mrchapp/gstreamer,drothlis/gstreamer,cablelabs/gstreamer,lovebug356/gstreamer,mparis/gstreamer,mrchapp/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,ensonic/gstreamer,Distrotech/gstreamer,mparis/gstreamer,magcius/gstreamer,lovebug356/gstreamer,magcius/gstreamer,ylatuya/gstreamer,cfoch/gstreamer,mrchapp/gstreamer,ahmedammar/platform_external_gst_gstreamer,mparis/gstreamer,ylatuya/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,drothlis/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,lovebug356/gstreamer,ylatuya/gstreamer,ylatuya/gstreamer,ensonic/gstreamer,collects/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,collects/gstreamer,collects/gstreamer,cablelabs/gstreamer,krieger-od/gstreamer