initialize r->cache->file.fd with NGX_INVALID_FILE

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 2fefe09..4dc2b57 100644
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -161,6 +161,7 @@
 
     r->cache = c;
     c->file.log = r->connection->log;
+    c->file.fd = NGX_INVALID_FILE;
 
     return NGX_OK;
 }