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/android/SDL_androidaudio.c

@@ -44,6 +44,7 @@ AndroidAUD_OpenDevice(_THIS, void *handle, const char *devname, int iscapture)
         return SDL_SetError("Capture not supported on Android");
     }
 
+    /* !!! FIXME: higher level will prevent this now. Lose this check (and global?). */
     if (audioDevice != NULL) {
         return SDL_SetError("Only one audio device at a time please!");
     }
@@ -115,10 +116,7 @@ AndroidAUD_CloseDevice(_THIS)
     Android_JNI_CloseAudioDevice();
 
     if (audioDevice == this) {
-        if (audioDevice->hidden != NULL) {
-            SDL_free(this->hidden);
-            this->hidden = NULL;
-        }
+        SDL_free(this->hidden);
         audioDevice = NULL;
     }
 }