backport of http://drupal.org/cvs?commit=90029 (added check to prevent "notice: Trying to get property of non-object ..." error message)
removed unused variable

solved regression bug introduced by http://drupal.org/cvs?commit=83921: font settings were moved to fieldset, but image_captcha_settings_form_pre_render() was not adapted to this

#177284: by  Rob Loach: usage of absolute path of ttf font file in GD ttf functions (some versions of GD don't work with relative paths)