Userid: ngx_http_get_indexed_variable() error handling.

When evaluating a mapped $reset_uid variable in the userid filter,
if get_handler set to ngx_http_map_variable() returned an error,
this previously resulted in a NULL pointer dereference.

src/http/modules/ngx_http_userid_filter_module.c

diff --git a/src/http/modules/ngx_http_userid_filter_module.c b/src/http/modules/ngx_http_userid_filter_module.c
index 0dbacba..a1a5493 100644
--- a/src/http/modules/ngx_http_userid_filter_module.c
+++ b/src/http/modules/ngx_http_userid_filter_module.c
@@ -472,6 +472,10 @@
 
         vv = ngx_http_get_indexed_variable(r, ngx_http_userid_reset_index);
 
+        if (vv == NULL || vv->not_found) {
+            return NGX_ERROR;
+        }
+
         if (vv->len == 0 || (vv->len == 1 && vv->data[0] == '0')) {
 
             if (conf->mark == '\0'