glib: Make sure GLIB_OLD_LOG_API is a NUL-terminated string

Every usage in GLib ensures this but theoretically external code might
pass something else. As this is only meant to be used internally from
GLib, don't support the other case but at least avoid potential out of
bound reads.

glib/gmessages.c

@@ -2625,6 +2625,7 @@ log_is_old_api (const GLogField *fields,
 {
   return (n_fields >= 1 &&
           g_strcmp0 (fields[0].key, "GLIB_OLD_LOG_API") == 0 &&
+          fields[0].length < 0 &&
           g_strcmp0 (fields[0].value, "1") == 0);
 }