null pointer segfaults
 dont print redundant spam
 dont print prefix if reference==NULL
 class -> av_class
 dont copy AVClass to every object, its a waste of memory and not a good idea at all


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@2841 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b