audio: changed some SDL_memset() calls to SDL_zero(), other minor corrections.

src/audio/emscripten/SDL_emscriptenaudio.c

@@ -171,7 +171,7 @@ Emscripten_OpenDevice(_THIS, void *handle, const char *devname, int iscapture)
     if (this->hidden == NULL) {
         return SDL_OutOfMemory();
     }
-    SDL_memset(this->hidden, 0, (sizeof *this->hidden));
+    SDL_zerop(this->hidden);
 
     /* based on parts of library_sdl.js */