[HTML5] Add checks to Gamepad API events.

In some conditions the events might be generated even when the `gamepad`
object is not accessible due to Security Context requirements.
This commit adds a check to avoid firing the handler in those cases.

platform/javascript/js/libs/library_godot_input.js

@@ -104,10 +104,14 @@ const GodotInputGamepads = {
 				}
 			}
 			GodotEventListeners.add(window, 'gamepadconnected', function (evt) {
-				add(evt.gamepad);
+				if (evt.gamepad) {
+					add(evt.gamepad);
+				}
 			}, false);
 			GodotEventListeners.add(window, 'gamepaddisconnected', function (evt) {
-				onchange(evt.gamepad.index, 0);
+				if (evt.gamepad) {
+					onchange(evt.gamepad.index, 0);
+				}
 			}, false);
 		},