ssh: fix copy and paste error

Doesn't matter because contentKey and lengthKey are the same length,
but we should really be looping over lengthKey here.

Change-Id: Ic7af3c553eb4382239bec1c030e77c7f5baa0662
Reviewed-on: https://go-review.googlesource.com/109735
Run-TryBot: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/cipher.go

@@ -659,7 +659,7 @@ func newChaCha20Cipher(key, unusedIV, unusedMACKey []byte, unusedAlgs directionA
 	for i := range c.contentKey {
 		c.contentKey[i] = binary.LittleEndian.Uint32(key[i*4 : (i+1)*4])
 	}
-	for i := range c.contentKey {
+	for i := range c.lengthKey {
 		c.lengthKey[i] = binary.LittleEndian.Uint32(key[(i+8)*4 : (i+9)*4])
 	}
 	return c, nil