buffer: fix the _get_mapped function
buffer: fix the _get_mapped function

Fix the internal _get_mapped function. gst_memory_make_mapped() takes ownership
of the memory so we need to keep an additional ref until we are done.

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