small fix for 1.5.1

If to use in a template:

<!-- IFDEF: USER_SURNAME -->
Users surname: {USER_SURNAME}
<!-- ELSE -->
<i>Users surname is not defined</i>
<!-- ENDIF -->

php generates the Notice

fix :
in function value_defined
find
if ($field{0}=='.') $field = substr($field, 1);

replace
if (isset($field{0}) && $field{0}=='.') $field = substr($field, 1);

OK, we will verify and fix it.