vorbis_parser: Move vp check to avoid a null pointer dereference

CC: libav-stable@libav.org
Bug-Id: CID 1251347

libavcodec/vorbis_parser.c

@@ -303,9 +303,9 @@ static int vorbis_parse(AVCodecParserContext *s1, AVCodecContext *avctx,
 
     if (!s->vp && avctx->extradata && avctx->extradata_size) {
         s->vp = av_vorbis_parse_init(avctx->extradata, avctx->extradata_size);
+    }
     if (!s->vp)
         goto end;
-    }
 
     if ((duration = av_vorbis_parse_frame(s->vp, buf, buf_size)) >= 0)
         s1->duration = duration;