Don't throw an exception on SDL_ThreadID()

This function is called by MESS as part of the SDL window initialization and the result is stored in a variable, but not really used for anything if multithreading is disabled (which it is in the Emscripten case). Currently we are stubbing it out in a --post-js file but might as well have it upstream in case other projects do the same thing. There's still a throw in SDL_CreateThread() if you actually try to use the threading functionality.