Fix IPv6 multicast address record

[gpotter2]

fixes #1260

According to page 20 of https://tools.ietf.org/html/rfc3810, it also seems that we build the packet correctly, even though wireshark fails when using auxiliary data.

[codecov-io]

Codecov Report

Merging #1261 into master will increase coverage by 0.79%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1261      +/-   ##
==========================================
+ Coverage   83.98%   84.78%   +0.79%     
==========================================
  Files         159      160       +1     
  Lines       37546    38299     +753     
==========================================
+ Hits        31532    32470     +938     
+ Misses       6014     5829     -185

Impacted Files	Coverage Δ
scapy/layers/inet6.py	85.84% <ø> (+0.43%)	⬆️
scapy/layers/tls/record.py	91.2% <0%> (-0.88%)	⬇️
scapy/asn1/ber.py	82% <0%> (-0.29%)	⬇️
scapy/config.py	85.12% <0%> (-0.28%)	⬇️
scapy/arch/windows/__init__.py	79.01% <0%> (ø)
scapy/fields.py	90.4% <0%> (+0.08%)	⬆️
scapy/main.py	71.51% <0%> (+0.63%)	⬆️
scapy/modules/nmap.py	97.01% <0%> (+0.74%)	⬆️
scapy/supersocket.py	79.22% <0%> (+0.86%)	⬆️
... and 12 more