fix(ipv6-opts): zero-length SMF_DPD option parses as H-DPD by reading the next option's type octet #1200

Describe the bug
In HOPOPT and IPv6-Opts, an SMF_DPD option with Opt Data Len 0 parses as H-DPD instead of being rejected, so it cannot be rebuilt.

The H-DPD/I-DPD bit is taken from the forward-matched next octet, which here belongs to the following option. That octet is read twice: once as this option's mode bit, and again as the type of the next option. The parsed option reports length=2 with hav=b''.

To Reproduce
On origin/main (630f0a581):

• HOPOPT(io.BytesIO(bytes.fromhex('3b00080080000000'))) parses SMFHashBasedDPDOption(length=2, hav=b'') followed by UnassignedOption(type=0x80).
• HOPOPT.from_data(p.info) then raises ProtocolError: … empty Hash Assist Value (HAV).
• The I-DPD form with length 0 already raises FieldValueError: invalid SMF I-DPD option length: 0.

Expected behavior
A zero-length SMF_DPD option raises FieldValueError in both modes, so a malformed option is never parsed into something that cannot be rebuilt.

Found during the #1194 cross-review. I re-ran it on main.