audio: Fixed potential race condition.

We need to check if the device is ready to close before releasing the lock,
in case other things are messing with the list of logical devices.

src/audio/SDL_audio.c

@@ -1353,9 +1353,12 @@ void SDL_CloseAudioDevice(SDL_AudioDeviceID devid)
         SDL_AudioDevice *device = logdev->physical_device;
         DestroyLogicalAudioDevice(logdev);
 
+        const SDL_bool close_physical = (device->logical_devices == NULL); // no more logical devices? Close the physical device, too.
+
         // !!! FIXME: we _need_ to release this lock, but doing so can cause a race condition if someone opens a device while we're closing it.
         SDL_UnlockMutex(device->lock);  // can't hold the lock or the audio thread will deadlock while we WaitThread it. If not closing, we're done anyhow.
-        if (device->logical_devices == NULL) {  // no more logical devices? Close the physical device, too.
+
+        if (close_physical) {
             ClosePhysicalAudioDevice(device);
         }