[dev] Signup.php - avoid some errors

[steve]

in lib/signup.php, if some fields are missing, we setup a default value

290a291,292
 >                 $label = isset($field['label']) ? $field['label'] : null;
 >                 $required = isset($field['required']) ? 
$field['required'] : false;
292c294
<                 $this->addVariable($field['label'], 'extra[' . 
$field_name . ']', $field['type'], $field['required'], $readonly, $desc, 
$field_params);
---
 >                 $this->addVariable($label, 'extra[' . $field_name . ']', 
$field['type'], $required, $readonly, $desc, $field_params);