vorbis_analysis_headerout calls oggpack_writeclear with uninitialized ogb if !v->backend_state

From Mozilla Bug 550184:

https://bugzilla.mozilla.org/show_bug.cgi?id=550184

567      int vorbis_analysis_headerout(vorbis_dsp_state *v,
568                                    vorbis_comment *vc,
569                                    ogg_packet *op,
570                                    ogg_packet *op_comm,
571                                    ogg_packet *op_code){

574        oggpack_buffer opb;
575        private_state *b=v->backend_state;

577        if(!b){
578          ret=OV_EFAULT;
579          goto err_out;

630       err_out:
631        oggpack_writeclear(&opb);