basesink: take timestamp later
basesink: take timestamp later

Make sure we don't accidentally cast a bufferlist of a buffer and try to take
the timestamp of it.

Refixes #585960

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