buddy: Assert that there is anything to allocate in alloc_async

Ref #53

src/buddy.c

@@ -2003,6 +2003,7 @@ BUDDYF(alloc_async)(struct buddy_reqs *reqs)
 
 	buddy = reqs->buddy;
 
+	assert(w->alloced < reqs->n);
 	assert(w->finid < reqs->n);
 
 	AZ(pthread_mutex_lock(&reqs->buddy->map_mtx));