x/net/ipv4: missing error handling in the Header.Marshal method

[perillo]

In the ipv4.Header.Marshal method, the code does not handle the possible error returned by h.Src.To4(). On the other hand, the possible error returned by h.Dst.To4() is handled correctly.

Another small issue is that the slice returned by To4() is still sliced (ip[:net.IPv4len]); this is unnecessary.

See https://github.com/golang/net/blob/master/ipv4/header.go#L92

[cagedmantis]

/cc @mikioh @ianlancetaylor