Emscripten: Fixed sending SDL_JOYDEVICEADDED events with an index out of range.

SDL_JOYDEVICEADDED events must contain the device index which is a value between
0 and the number of connected joysticks. The old implementation included a value
based on the instance id instead. It worked in some cases because the values are
similar initially. But after disconnecting joysticks this is no more the case
and the always increasing instance id becomes larger than number of joysticks.

src/joystick/emscripten/SDL_sysjoystick.c

@@ -112,7 +112,7 @@ Emscripten_JoyStickConnected(int eventType, const EmscriptenGamepadEvent *gamepa
     event.type = SDL_JOYDEVICEADDED;
 
     if (SDL_GetEventState(event.type) == SDL_ENABLE) {
-        event.jdevice.which = item->device_instance - 1;
+        event.jdevice.which = item->index;
         if ( (SDL_EventOK == NULL) ||
              (*SDL_EventOK) (SDL_EventOKParam, &event) ) {
             SDL_PushEvent(&event);