private static ByteBuf safeReadBytes(StreamDecoderInput in, int length) {
        if (length == 0) {
            return Unpooled.EMPTY_BUFFER;
        } else {
            return in.readBytes(length);
        }
    }
Reads the next part body content.
Returns {@link #NEED_MORE} if more data is required and no body content can be returned.
core/src/main/java/com/linecorp/armeria/common/multipart/MimeParser.java