fix segfault

src/http/modules/ngx_http_charset_filter_module.c

diff --git a/src/http/modules/ngx_http_charset_filter_module.c b/src/http/modules/ngx_http_charset_filter_module.c
index 8c9fad6..f7d9906 100644
--- a/src/http/modules/ngx_http_charset_filter_module.c
+++ b/src/http/modules/ngx_http_charset_filter_module.c
@@ -1462,6 +1462,12 @@
         return NGX_CONF_OK;
     }
 
+    if (conf->source_charset >= NGX_HTTP_CHARSET_VAR
+        || conf->charset >= NGX_HTTP_CHARSET_VAR)
+    {
+        return NGX_CONF_OK;
+    }
+
     mcf = ngx_http_conf_get_module_main_conf(cf,
                                              ngx_http_charset_filter_module);
     recode = mcf->recodes.elts;