RFC7112 requires that the first fragment in a fragment chain contains a valid
"When a host fragments an IPv6 datagram, it MUST include the entire
IPv6 Header Chain in the First Fragment."
Check that the first fragment (frag offset = 0) has a next header which is an
upper layer header and check that there is enough payload in the fragment to
evaluate the upper layer header.
Drop any fragments that have already arrived for this fragment chain if header