Function read_packet_length 

pub fn read_packet_length(
    len_of_len: u8,
    bytes: &[u8],
) -> Result<Option<u32>, Error>

Reads the variable-length packet length from the bytes following the 2-byte mandatory header.

len_of_len is the value from the header’s Length of Length field. bytes should start at the first byte after the mandatory header.