forms: maxlength of 0 means disable maxlength

Instead, previously, a maxlength of zero effectively disabled the field,
because it prevented any user input into the field.

include/class.forms.php

@@ -954,7 +954,7 @@ class TextboxWidget extends Widget {
         $config = $this->field->getConfiguration();
         if (isset($config['size']))
             $size = "size=\"{$config['size']}\"";
-        if (isset($config['length']))
+        if (isset($config['length']) && $config['length'])
             $maxlength = "maxlength=\"{$config['length']}\"";
         if (isset($config['classes']))
             $classes = 'class="'.$config['classes'].'"';