util/u_queue: skip util_queue_finish if num_threads is 0

This fixes a deadlock in pthread_barrier_destroy.

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/util/u_queue.c

@@ -647,6 +647,13 @@ util_queue_finish(struct util_queue *queue)
     * wait for it exclusively.
     */
    mtx_lock(&queue->finish_lock);
+
+   /* The number of threads can be changed to 0, e.g. by the atexit handler. */
+   if (!queue->num_threads) {
+      mtx_unlock(&queue->finish_lock);
+      return;
+   }
+
    fences = malloc(queue->num_threads * sizeof(*fences));
    util_barrier_init(&barrier, queue->num_threads);