Revert "audio: don't block on lock in ao_read_data"

It was found that this causes issues with at least ao_coreaudio,
essentially revealing a way bigger issue:
Some AOs don't check for 0 and/or have no way to deal with short writes.
Someone will have to figure out a fix later but get rid of the direct
cause for now.

This reverts commit ae908a70ce.

audio/out/buffer.c

@@ -184,8 +184,7 @@ 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);
 
-    if (pthread_mutex_trylock(&p->lock))
-        return 0;
+    pthread_mutex_lock(&p->lock);
 
     int pos = read_buffer(ao, data, samples, &(bool){0});