buffer: don't clear TAG on NULL buffer
buffer: don't clear TAG on NULL buffer

When the buffer fails to copy, don't clear the TAG on the NULL pointer.

cfoch/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,centricular/gstreamer,cfoch/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,mparis/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,mparis/gstreamer,mparis/gstreamer,surround-io/gstreamer,krichter722/gstreamer,mparis/gstreamer,krichter722/gstreamer,centricular/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,krichter722/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,centricular/gstreamer