Fixed bug #728

I am using a headset on my monitor as primary sound output device. From time to
time it happens that I switch off my monitor when I leave my computer and
switch it on when I come back.
When doing so while wesnoth is running I get no sound back when I switch on my
monitor. There is probably good reason for that. Therefore I wanted to exit and
restart wesnoth. Unfortunately, wesnoth always hangs in this situation. It
looks like wesnoth can not be exited when the primary sound device vanished.
I created a backtrace of this situation (please see attachment).

--HG--
branch : SDL-1.2

src/audio/alsa/SDL_alsa_audio.c

@@ -323,7 +323,7 @@ static void ALSA_PlayAudio(_THIS)
 
 	frames_left = ((snd_pcm_uframes_t) this->spec.samples);
 
-	while ( frames_left > 0 ) {
+	while ( frames_left > 0 && this->enabled ) {
 		status = SDL_NAME(snd_pcm_writei)(pcm_handle, sample_buf, frames_left);
 		if ( status < 0 ) {
 			if ( status == -EAGAIN ) {