crypto/rsa: don't correct private exponent unless needed.

At some point in the past, I believe the GCD algorithm was setting d to
be negative. The RSA code has been correcting that ever since but, now,
it appears to have changed and the correction isn't needed.

Having d be too large is harmless, it's just a little odd and I
happened to notice.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7948044

src/pkg/crypto/rsa/rsa.go

@@ -203,7 +203,9 @@ NextSetOfPrimes:
 		g.GCD(priv.D, y, e, totient)
 
 		if g.Cmp(bigOne) == 0 {
+			if priv.D.Sign() < 0 {
 				priv.D.Add(priv.D, totient)
+			}
 			priv.Primes = primes
 			priv.N = n
 

src/pkg/crypto/rsa/rsa_test.go

@@ -93,6 +93,9 @@ func testKeyBasics(t *testing.T, priv *PrivateKey) {
 	if err := priv.Validate(); err != nil {
 		t.Errorf("Validate() failed: %s", err)
 	}
+	if priv.D.Cmp(priv.N) > 0 {
+		t.Errorf("private exponent too large")
+	}
 
 	pub := &priv.PublicKey
 	m := big.NewInt(42)