Drop 0-byte IPv6 fragments.

Currently, we process IPv6 fragments with 0 bytes of payload, add them
to the reassembly queue, and do not recognize them as duplicating or
overlapping with adjacent 0-byte fragments. An attacker can exploit this
to create long fragment queues.

There is no legitimate reason for a fragment with no payload. However,
because IPv6 packets with an empty payload are acceptable, allow an
"atomic" fragment with no payload.

Reviewed by: jhb
Security: FreeBSD-SA-18:10.ip
Security: CVE-2018-6923