audio: Clean up some CloseDevice() interface details.

- It's now always called if device->hidden isn't NULL, even if OpenDevice()
  failed halfway through. This lets implementation code not have to clean up
  itself on every possible failure point; just return an error and SDL will
  handle it for you.

- Implementations can assume this->hidden != NULL and not check for it.

- implementations don't have to set this->hidden = NULL when done, because
  the caller is always about to free(this).

- Don't reset other fields that are in a block of memory about to be free()'d.

- Implementations all now free things like internal mix buffers last, after
  closing devices and such, to guarantee they definitely aren't in use anymore
  at the point of deallocation.

src/audio/emscripten/SDL_emscriptenaudio.c

@@ -136,16 +136,8 @@ HandleAudioProcess(_THIS)
 static void
 Emscripten_CloseDevice(_THIS)
 {
-    if (this->hidden != NULL) {
-        if (this->hidden->mixbuf != NULL) {
-            /* Clean up the audio buffer */
-            SDL_free(this->hidden->mixbuf);
-            this->hidden->mixbuf = NULL;
-        }
-
-        SDL_free(this->hidden);
-        this->hidden = NULL;
-    }
+    SDL_free(this->hidden->mixbuf);
+    SDL_free(this->hidden);
 }
 
 static int