avcodec/mpeg4video_parser: fix spurious extradata parse warnings

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg4video_parser.c

@@ -88,7 +88,7 @@ static int mpeg4_decode_header(AVCodecParserContext *s1, AVCodecContext *avctx,
     if (avctx->extradata_size && pc->first_picture) {
         init_get_bits(gb, avctx->extradata, avctx->extradata_size * 8);
         ret = ff_mpeg4_decode_picture_header(dec_ctx, gb);
-        if (ret < 0)
+        if (ret < -1)
             av_log(avctx, AV_LOG_WARNING, "Failed to parse extradata\n");
     }