audio: don't free uninitialized audio_buffer

free(NULL) isn't explicitly forbidden, but isn't exactly good style.
Check the rare case that the audio buffer isn't initialized yet in
closeAudioDevice().  In this case, we also don't have to call
flushAudioBuffer().

src/audio.c

@@ -386,11 +386,12 @@ void closeAudioDevice(void)
 {
 	unsigned int i;
 
-	flushAudioBuffer();
-
-	free(audio_buffer.buffer);
-	audio_buffer.buffer = NULL;
-	audio_buffer.size = 0;
+	if (audio_buffer.buffer != NULL) {
+		flushAudioBuffer();
+		free(audio_buffer.buffer);
+		audio_buffer.buffer = NULL;
+		audio_buffer.size = 0;
+	}
 
 	for (i = 0; i < audioOutputArraySize; ++i) {
 		if (audioDeviceStates[i] == DEVICE_ON)