Merge some cherry-picked fixes originating in OpenBSD
    
    Check whether the version field is available before looking at it.
    While we're at it, use ND_TCHECK(), rather than a hand-rolled check, to
    check whether we have the full fixed-length portion of the IPv4 header.
    
    commit c67afe913011138a2504ec4d3d423b48e73b12f3
    
    Do more length checking. From OpenBSD.
    
    commit d7516761f9c4877bcb05bb6543be3543e165249