Using a controller in HTML5 causes error

When using a controller in HTML5, the project crashes within the browser. However, this does not occur on Google Chrome. FireFox/Nightly crashes on launch on as well as packaging and running the project there - if you use a controller.

This was tested on both Binary and Source.

1. Open any template within the Editor
2. File > Package > HTML5
3. Plug in an Xbox 360 controller
4. Run HTML5LaunchHelper.exe
5. Open FireFox/FireFox Nightly "Http://localhost:8000/PROJECTNAME.html
6. After the game loads, use the controller

Result: The project crashes in HTML5
Expected: The game should work properly with a controller, as it does on Google Chrome.

Expression (Key.IsValid()) failed in D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.10\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:4912!
Check console for details.

FSlateApplication::OnControllerButtonReleased?5()
undefined?T()
FHTML5InputInterface::SendControllerEvents()
FHTML5Application::PollGameDeviceState(float)
FSlateApplication::PollGameDeviceState()
FEngineLoop::Tick()
HTML5_Tick()
dynCall_v

Fix UE-51650

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