Emscripten: Fixed opening previously closed joystick.

src/joystick/emscripten/SDL_sysjoystick.c

@@ -377,6 +377,8 @@ SDL_SYS_JoystickUpdate(SDL_Joystick * joystick)
 void
 SDL_SYS_JoystickClose(SDL_Joystick * joystick)
 {
+    SDL_joylist_item *item = (SDL_joylist_item *) joystick->hwdata;
+    item->joystick = NULL;
 }
 
 /* Function to perform any system-specific joystick related cleanup */