Put following function into your module and you can use it for form elements through a #element_validate entry.
<?php
function element_validate_valid_path($element, &$form_state) {
$value = $element['#value'];
if (!drupal_valid_path($value)) {
form_error($element, t('%name must be a valid path.', array('%name' => $element['#title'])));
}
}