crypto/rsa: drop uneeded parameter in modInverse

The current modInverse implementation allocates a big.Int
for the second parameter of GCD, while only the first is needed.
This is unnecessary and can lead to a speed up for optimizations
of GCD where the second parameter is not calculated at all.

Change-Id: I3f042e140ff643311bc3d0b8d192992d4d2c4c70
Reviewed-on: https://go-review.googlesource.com/50531
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filosottile.wiki@gmail.com>
Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/rsa/rsa.go

@@ -424,8 +424,7 @@ var ErrVerification = errors.New("crypto/rsa: verification error")
 func modInverse(a, n *big.Int) (ia *big.Int, ok bool) {
 	g := new(big.Int)
 	x := new(big.Int)
-	y := new(big.Int)
-	g.GCD(x, y, a, n)
+	g.GCD(x, nil, a, n)
 	if g.Cmp(bigOne) != 0 {
 		// In this case, a and n aren't coprime and we cannot calculate
 		// the inverse. This happens because the values of n are nearly