core: wbxmldecoder: simplify getOpaque
core: wbxmldecoder: simplify getOpaque

use stream_get_contents()

Signed-off-by: Etienne CHAMPETIER <d24370f93c848f9b9e9043ebb082d433901eaa60@fiducial.net>

Freeaqingme/Z-Push-contrib,RavenB/Z-Push-contrib,mta59066/Z-Push-contrib,fmbiete/Z-Push-contrib