gnrc_icmpv6_error: check IPv6 header before starting to build

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)