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/nacl/SDL_naclaudio.c

@@ -43,8 +43,6 @@
 #define SAMPLE_FRAME_COUNT 4096
 
 /* Audio driver functions */
-static int NACLAUD_OpenDevice(_THIS, void *handle, const char *devname, int iscapture);
-static void NACLAUD_CloseDevice(_THIS);
 static void nacl_audio_callback(void* samples, uint32_t buffer_size, PP_TimeDelta latency, void* data);
 
 /* FIXME: Make use of latency if needed */