omit unnecessary test, because NGX_ERROR == -1,
but r->headers_out.last_modified_time != -1 at this point

src/http/modules/ngx_http_not_modified_filter_module.c

diff --git a/src/http/modules/ngx_http_not_modified_filter_module.c b/src/http/modules/ngx_http_not_modified_filter_module.c
index dc62636..ce4b55c 100644
--- a/src/http/modules/ngx_http_not_modified_filter_module.c
+++ b/src/http/modules/ngx_http_not_modified_filter_module.c
@@ -70,7 +70,7 @@
      * I think that the equality of the dates is correcter
      */
 
-    if (ims != NGX_ERROR && ims == r->headers_out.last_modified_time) {
+    if (ims == r->headers_out.last_modified_time) {
         r->headers_out.status = NGX_HTTP_NOT_MODIFIED;
         r->headers_out.content_type.len = 0;
         ngx_http_clear_content_length(r);