Increase EventDriver::__destruct robustness (#358)

Preventing a double free of an event due to destructors freeing the event during freeing in EventDriver destructor.

lib/Loop/EventDriver.php

@@ -164,16 +164,17 @@ public static function isSupported(): bool
      */
     public function __destruct()
     {
-        foreach ($this->events as $event) {
+        // Unset here, otherwise $event->del() in the loop may fail with a warning, because __destruct order isn't defined.
+        // Related https://github.com/amphp/amp/issues/159.
+        $events = $this->events;
+        $this->events = [];
+
+        foreach ($events as $event) {
             if ($event !== null) { // Events may have been nulled in extension depending on destruct order.
                 $event->free();
             }
         }
 
-        // Unset here, otherwise $event->del() fails with a warning, because __destruct order isn't defined.
-        // See https://github.com/amphp/amp/issues/159.
-        $this->events = [];
-
         // Manually free the loop handle to fully release loop resources.
         // See https://github.com/amphp/amp/issues/177.
         if ($this->handle !== null) {