avformat/yuv4mpegdec: return proper error

The header is not invalid in this case, but ffmpeg still doesn't
support it.

libavformat/yuv4mpegdec.c

@@ -151,6 +151,7 @@ static int yuv4_read_header(AVFormatContext *s)
             case 'm':
                 av_log(s, AV_LOG_ERROR, "YUV4MPEG stream contains mixed "
                        "interlaced and non-interlaced frames.\n");
+                return AVERROR(ENOTSUP);
             default:
                 av_log(s, AV_LOG_ERROR, "YUV4MPEG has invalid header.\n");
                 return AVERROR(EINVAL);