[dev.boringcrypto] crypto/rsa: fix boringFakeRandomBlind to work with (*big.Int).ModInverse

http://golang.org/cl/108996 removed the local modInverse and its call in
decrypt in favor of (*big.Int).ModInverse. boringFakeRandomBlind copies
decrypt, so it needs to be updated as well.

Change-Id: I59a6c17c2fb9cc7f38cbb59dd9ed11846737d220
Reviewed-on: https://go-review.googlesource.com/113676
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/rsa/boring.go

@@ -147,6 +147,7 @@ func boringFakeRandomBlind(random io.Reader, priv *PrivateKey) {
 	boring.UnreachableExceptTests()
 
 	// Copied from func decrypt.
+	ir := new(big.Int)
 	for {
 		r, err := rand.Int(random, priv.N)
 		if err != nil {
@@ -155,8 +156,8 @@ func boringFakeRandomBlind(random io.Reader, priv *PrivateKey) {
 		if r.Cmp(bigZero) == 0 {
 			r = bigOne
 		}
-		_, ok := modInverse(r, priv.N)
-		if ok {
+		ok := ir.ModInverse(r, priv.N)
+		if ok != nil {
 			break
 		}
 	}