While 485dbd1fda (from #12175) was right
in assuming that the for most ICMPv6 error messages the originating
packet's destination address must not be a multicast, this is not the
case for _all_ ICMPv6 error messages (see [RFC 4443], section 2.4(e.3)).
Additionally, 485dbd1fda removed the
check for the source address ([RFC 4443], section 2.4(e.6)), which this
PR re-adds.
[RFC 4443]: https://tools.ietf.org/html/rfc4443#section-2.4
before this commit the src address was checked for multicast, but the dst address should be checked. Therefore udp multicast packets would be flooded back to the src as ICMPv6 error, as not all nodes had a UDP receiver registered.
Check for:
- if it exists (critical error condition -- non-IPv6 headers should
not trigger these functions) => assert
- if it has a multicast source (that shouldn't really happen but
people might try weird stuff ;-)
- if it has an unspecified source (can't determine receiver of error
message => don't send it, don't build it)
