audio: don't block on lock in ao_read_data

ao_read_data() is used by pull AOs potentially from threads managed by
external libraries.  These threads can be sensitive to blocking.
For example the pipewire ao is using a realtime thread for the
callbacks.

audio/out/buffer.c

@@ -184,7 +184,8 @@ int ao_read_data(struct ao *ao, void **data, int samples, int64_t out_time_ns)
     struct buffer_state *p = ao->buffer_state;
     assert(!ao->driver->write);
 
-    pthread_mutex_lock(&p->lock);
+    if (pthread_mutex_trylock(&p->lock))
+        return 0;
 
     int pos = read_buffer(ao, data, samples, &(bool){0});