Avoid panic opportunity for TCP keep-alive timers

There is a panic opportunity if the timer fires but the socket is
going through cleanup. In this case `e.route` might go away but the
timer handler for keep-alive continues to execute and causes panics.
The fix is to return early from the handler if we find out the route
is already removed.

PiperOrigin-RevId: 619268432

pkg/tcpip/transport/tcp/connect.go

@@ -1292,6 +1292,11 @@ func (e *Endpoint) handleSegmentLocked(s *segment) (cont bool, err tcpip.Error)
 func (e *Endpoint) keepaliveTimerExpired() tcpip.Error {
 	userTimeout := e.userTimeout
 
+	// If the route is not ready or already cleaned up, then we don't need to
+	// send keepalives.
+	if e.route == nil {
+		return nil
+	}
 	e.keepalive.Lock()
 	if !e.SocketOptions().GetKeepAlive() || e.keepalive.timer.isUninitialized() || !e.keepalive.timer.checkExpiration() {
 		e.keepalive.Unlock()