do not free unused cache node if cache min_uses > 1,
the bug has been introduced in r3695, r3708, r3711

src/http/ngx_http_file_cache.c

diff --git a/src/http/ngx_http_file_cache.c b/src/http/ngx_http_file_cache.c
index 133a52e..4d84734 100644
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -932,7 +932,7 @@
         fcn->valid_msec = c->valid_msec;
         fcn->error = c->error;
 
-    } else if (!fcn->exists && fcn->count == 0) {
+    } else if (!fcn->exists && fcn->count == 0 && c->min_uses == 1) {
         ngx_queue_remove(&fcn->queue);
         ngx_rbtree_delete(&cache->sh->rbtree, &fcn->node);
         ngx_slab_free_locked(cache->shpool, fcn);