Fix crash in SDL audio thread, by Juha Kuikka

Wrong audio spec structure was populated with the internal callback, causing the audio thread to call a NULL pointer.

src/audio/SDL_audio.c

@@ -1344,8 +1344,8 @@ open_audio_device(const char *devname, int iscapture,
             SDL_SetError("Couldn't create audio buffer queue");
             return 0;
         }
-        device->spec.callback = iscapture ? SDL_BufferQueueFillCallback : SDL_BufferQueueDrainCallback;
-        device->spec.userdata = device;
+        device->callbackspec.callback = iscapture ? SDL_BufferQueueFillCallback : SDL_BufferQueueDrainCallback;
+        device->callbackspec.userdata = device;
     }
 
     /* Allocate a scratch audio buffer */