go.crypto/ssh: allow zero sized window adjustments

The RFC doesn't prohibit zero sized window adjustments and
there is evidence of well known clients using them in the
wild.

R=agl, kardianos, gustav.paul
CC=golang-dev
https://golang.org/cl/6209082

ssh/common.go

@@ -300,8 +300,9 @@ type window struct {
 // add adds win to the amount of window available
 // for consumers.
 func (w *window) add(win uint32) bool {
+	// a zero sized window adjust is a noop.
 	if win == 0 {
-		return false
+		return true
 	}
 	w.L.Lock()
 	if w.win+win < win {