Rewrite: fixed "return" directive without response text.

Previously, the response text wasn't initialized and the rewrite module
was sending response body set to NULL.

Found with UndefinedBehaviorSanitizer (pointer-overflow).

Signed-off-by: Piotr Sikora <piotr@aviatrix.com>

src/http/modules/ngx_http_rewrite_module.c

@@ -489,6 +489,7 @@ ngx_http_rewrite_return(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
         }
 
         if (cf->args->nelts == 2) {
+            ngx_str_set(&ret->text.value, "");
             return NGX_CONF_OK;
         }