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