ssh: fix reset{Read,Write}Thresholds for initial setup

Fixes a nil pointer dereference that slipped through buildbots because
it was introduced by the last two commits.

Change-Id: Ib269e910956cd8b3b46e217b03fde1b61572260a
Reviewed-on: https://go-review.googlesource.com/40530
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/handshake.go

@@ -245,6 +245,8 @@ func (t *handshakeTransport) resetWriteThresholds() {
 		t.writeBytesLeft = int64(t.config.RekeyThreshold)
 	} else if t.algorithms != nil {
 		t.writeBytesLeft = t.algorithms.w.rekeyBytes()
+	} else {
+		t.writeBytesLeft = 1 << 30
 	}
 }
 
@@ -355,8 +357,10 @@ func (t *handshakeTransport) resetReadThresholds() {
 	t.readPacketsLeft = packetRekeyThreshold
 	if t.config.RekeyThreshold > 0 {
 		t.readBytesLeft = int64(t.config.RekeyThreshold)
-	} else {
+	} else if t.algorithms != nil {
 		t.readBytesLeft = t.algorithms.r.rekeyBytes()
+	} else {
+		t.readBytesLeft = 1 << 30
 	}
 }