[mod_auth] minor: adjust config validation

x-ref:
  "PVS-Studio Analysis Results"
  https://redmine.lighttpd.net/boards/3/topics/8459
  http://www.fly-server.ru/pvs-studio/lighttpd/

src/mod_auth.c

@@ -138,7 +138,7 @@ static int mod_auth_require_parse (server *srv, http_auth_require_t * const requ
                             "error value:", b, "error near:", str);
             return 0;
         }
-        if (p-1  == eq) {
+        if (eq[1] == '|' || eq[1] == '\0') {
             log_error_write(srv, __FILE__, __LINE__, "sssbss",
                             "error parsing auth.require 'require' field: missing token after '='",
                             "(expecting \"valid-user\" or \"user=a|user=b|group=g|host=h\").",