zero cache file length,
this fixes full cache purging and hogging CPU by the cache manager after this

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 42315f7..d000a9d 100644
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -448,6 +448,7 @@
     fcn->valid_sec = 0;
     fcn->uniq = 0;
     fcn->body_start = 0;
+    fcn->length = 0;
 
 done: