Fixed buffer overrun in adafruit_wiznet5k_dhcp.send_dhcp_message (non breaking)

[BiffoBear]

closes #68 Check length of MAC address is 6 bytes in DHCP.__init__()

    def __init__(
        self, eth, mac_address, hostname=None, response_timeout=30, debug=False
        ...
                # Prevent buffer overrun in send_dhcp_message()
        if len(mac_address) != 6:
            raise ValueError("The MAC address must be 6 bytes.")
        self._mac_address = mac_address

N.B. I don't have hardware to test this.

[tekktrik]

Looks good to me, thanks for the fix!