math/big: fix Exp when exponent is 1

Fixed bug that caused Exp(x, y, m) ( i.e. x**y (mod m) ) to return x
instead of x (mod m) when y == 1. See issue page on github for more
details.

Added test case

Fixes #9826

Change-Id: Ibabb58275a20c4231c9474199b7f1c10e54241ce
Reviewed-on: https://go-review.googlesource.com/8409Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/int_test.go

@@ -525,6 +525,7 @@ var expTests = []struct {
 	{"1234", "-1", "1", "0"},
 
 	// misc
+	{"5", "1", "3", "2"},
 	{"5", "-7", "", "1"},
 	{"-5", "-7", "", "1"},
 	{"5", "0", "", "1"},

src/math/big/nat.go

@@ -888,6 +888,13 @@ func (z nat) expNN(x, y, m nat) nat {
 	}
 	// y > 0
 
+	// x**1 mod m == x mod m
+	if len(y) == 1 && y[0] == 1 && len(m) != 0 {
+		_, z = z.div(z, x, m)
+		return z
+	}
+	// y > 1
+
 	if len(m) != 0 {
 		// We likely end up being as long as the modulus.
 		z = z.make(len(m))