Prevent loop from being run on shutdown if it has already been run

[mbonneau]

If a loop is passed into connect() and run prior to shutdown, Pawl starts it back up.

You can reproduce this behavior:

use function Ratchet\Client\connect;
use React\EventLoop\Factory;
use React\EventLoop\Timer\TimerInterface;

require __DIR__ . '/vendor/autoload.php';

$loop = Factory::create();

$loop->addPeriodicTimer(1, function (TimerInterface $timer) {
    static $x = 0;

    echo ++$x;

    if ($x === 5) {
        $timer->getLoop()->stop();
    }
});

$x = connect('ws://echo.socketo.me:9000', [], [], $loop);

$loop->run();

The expected output would be: 12345
The output without this change is: 123456789...