crypto/twofish: fix Reset index overflow bug.

Fixes #1919.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4530110

src/pkg/crypto/twofish/twofish.go

@@ -116,7 +116,7 @@ func (c *Cipher) Reset() {
 		c.k[i] = 0
 	}
 	for i := range c.s {
-		for j := 0; j < 265; j++ {
+		for j := 0; j < 256; j++ {
 			c.s[i][j] = 0
 		}
 	}