basesrc: provide fallback in case a create function doesn't know about provided buffers
basesrc: provide fallback in case a create function doesn't know about provided buffers

In 0.11 the caller may provide a buffer to be filled by the source to
pull_range/get_range/create, but it's easy to miss this new case when
porting code from 0.10. Provide fallback that copies the created data
into the provided buffer for now.

This makes oggdemux in pull-mode work with dataurisrc.

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