crypto/tls: check curve equation in ECDHE.

This change causes a TLS client and server to verify that received
elliptic curve points are on the expected curve. This isn't actually
necessary in the Go TLS stack, but Watson Ladd has convinced me that
it's worthwhile because it's pretty cheap and it removes the
possibility that some change in the future (e.g. tls-unique) will
depend on it without the author checking that precondition.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/115290046

key_agreement.go

@@ -292,6 +292,9 @@ func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Cert
 	if x == nil {
 		return nil, errClientKeyExchange
 	}
+	if !ka.curve.IsOnCurve(x, y) {
+		return nil, errClientKeyExchange
+	}
 	x, _ = ka.curve.ScalarMult(x, y, ka.privateKey)
 	preMasterSecret := make([]byte, (ka.curve.Params().BitSize+7)>>3)
 	xBytes := x.Bytes()
@@ -322,6 +325,9 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	if ka.x == nil {
 		return errServerKeyExchange
 	}
+	if !ka.curve.IsOnCurve(ka.x, ka.y) {
+		return errServerKeyExchange
+	}
 	serverECDHParams := skx.key[:4+publicLen]
 
 	sig := skx.key[4+publicLen:]