crypto/tls: don't check whether an ec point is on a curve twice

The processClientKeyExchange and processServerKeyExchange functions unmarshal an
encoded EC point and explicitly check whether the point is on the curve. The explicit
check can be omitted because elliptic.Unmarshal fails if the point is not on the curve
and the returned error would always be the same.

Fixes #20496

Change-Id: I5231a655eace79acee2737dd036a0c255ed42dbb
Reviewed-on: https://go-review.googlesource.com/44311
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Run-TryBot: Adam Langley <agl@golang.org>

key_agreement.go

@@ -319,13 +319,10 @@ func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Cert
 	if !ok {
 		panic("internal error")
 	}
-	x, y := elliptic.Unmarshal(curve, ckx.ciphertext[1:])
+	x, y := elliptic.Unmarshal(curve, ckx.ciphertext[1:]) // Unmarshal also checks whether the given point is on the curve
 	if x == nil {
 		return nil, errClientKeyExchange
 	}
-	if !curve.IsOnCurve(x, y) {
-		return nil, errClientKeyExchange
-	}
 	x, _ = curve.ScalarMult(x, y, ka.privateKey)
 	preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
 	xBytes := x.Bytes()
@@ -365,14 +362,10 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 		if !ok {
 			return errors.New("tls: server selected unsupported curve")
 		}
-
-		ka.x, ka.y = elliptic.Unmarshal(curve, publicKey)
+		ka.x, ka.y = elliptic.Unmarshal(curve, publicKey) // Unmarshal also checks whether the given point is on the curve
 		if ka.x == nil {
 			return errServerKeyExchange
 		}
-		if !curve.IsOnCurve(ka.x, ka.y) {
-			return errServerKeyExchange
-		}
 	}
 
 	sigAndHash := signatureAndHash{signature: ka.sigType}