Fix crash when using iblock option (when a warning is logged).

Patch by James Zern, jzern google

Originally committed as revision 25472 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/libvorbis.c

@@ -58,7 +58,7 @@ static const AVOption options[]={
 {"iblock", "Sets the impulse block bias", offsetof(OggVorbisContext, iblock), FF_OPT_TYPE_DOUBLE, 0, -15, 0, AV_OPT_FLAG_ENCODING_PARAM},
 {NULL}
 };
-static const AVClass class = { "libvorbis", NULL, options, LIBAVUTIL_VERSION_INT };
+static const AVClass class = { "libvorbis", av_default_item_name, options, LIBAVUTIL_VERSION_INT };
 
 static av_cold int oggvorbis_init_encoder(vorbis_info *vi, AVCodecContext *avccontext) {
     OggVorbisContext *context = avccontext->priv_data ;