undefined index: fee & calcfee

Proper checks are missing, see patch

Notice: Undefined index: fee in dibs_frontend_dibs_callback_ok() (line 524 of /builds/jenkins-stampsgl2_test-60/htdocs/profiles/stampsgl/modules/contrib/dibs/dibs.frontend.inc).

Notice: Undefined index: calcfee in dibs_frontend_redirect_form() (line 224 of /builds/jenkins-stampsgl2_test-60/htdocs/profiles/stampsgl/modules/contrib/dibs/dibs.frontend.inc).