fix r3708 and r3695: valid_sec is set only for caching error status codes

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 ad97f07..3bb7054 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->valid_sec == 0 && fcn->count == 0) {
+    } else if (!fcn->exists && fcn->count == 0) {
         ngx_queue_remove(&fcn->queue);
         ngx_rbtree_delete(&cache->sh->rbtree, &fcn->node);
         ngx_slab_free_locked(cache->shpool, fcn);