initialize use_event field in open file cache
the bug had been introduced in r2071

src/core/ngx_open_file_cache.c

diff --git a/src/core/ngx_open_file_cache.c b/src/core/ngx_open_file_cache.c
index 173980f..aa87fd2 100644
--- a/src/core/ngx_open_file_cache.c
+++ b/src/core/ngx_open_file_cache.c
@@ -360,6 +360,7 @@
 
     file->uses = 1;
     file->count = 0;
+    file->use_event = 0;
     file->event = NULL;
 
 add_event: