bytewriter: fix compiler warning
bytewriter: fix compiler warning

Some gcc versions warn about bytewriter writing to memory accessed
via a const guint8 pointer, despite our explicit cast to guint8 *.
Work around that by using an intermediary variable.

Fixes #598526.

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