fix "stalled cache updating" alert,
when non-cachable HEAD response did not not free an expired cache node

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 8f1ee42..22187a5 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2031,6 +2031,15 @@
             c->error = 1;
 
         } else {
+
+#if (NGX_HTTP_CACHE)
+
+            if (r->cache) {
+                ngx_http_file_cache_free(r->cache, u->pipe->temp_file);
+            }
+
+#endif
+
             ngx_http_upstream_finalize_request(r, u, rc);
             return;
         }