gallium/u_queue: set num_threads correctly if not all threads start

If i-th thread could not be created it means we have i threads,
not i+1, because we start from 0.

Fixes: 404d0d5 "gallium/u_queue: add an option to have multiple worker threads"
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/auxiliary/util/u_queue.c

@@ -236,7 +236,7 @@ util_queue_init(struct util_queue *queue,
             goto fail;
          } else {
             /* at least one thread created, so use it */
-            queue->num_threads = i+1;
+            queue->num_threads = i;
             break;
          }
       }