sdl_impl: Fix controller reconnection issues

It turns out that after a controller is disconnected, there is a chance that events from the previous controller are sent/processed after it has been disconnected.
This causes the previously disconnected controller to reappear as connected due to GetSDLJoystickBySDLID() emplacing this controller back to the map.

Fix this by only returning an SDLJoystick if and only if it exists in the map.