ao_alsa: fix an error check

Fixes #4188 as pointed out in the issue.

audio/out/ao_alsa.c

@@ -1121,7 +1121,7 @@ static int audio_wait(struct ao *ao, pthread_mutex_t *lock)
             return r;
 
         unsigned short revents;
-        snd_pcm_poll_descriptors_revents(p->alsa, fds, num_fds, &revents);
+        err = snd_pcm_poll_descriptors_revents(p->alsa, fds, num_fds, &revents);
         CHECK_ALSA_ERROR("cannot read poll events");
 
         if (revents & POLLERR)