Some more PHP Notice Fixes

referer: http://openemr.example.com/interface/main/left_nav.php
in /var/www/openemr/interface/billing/new_payment.php on line :
Fixes:
Undefined variable: payment_id in /var/www/openemr/interface/billing/new_payment.php on line 122
Undefined variable: hidden_patient_code in /var/www/openemr/interface/billing/new_payment.php on line 405
Undefined index: payment_id in /var/www/openemr/interface/billing/new_payment.php on line 122
Undefined index: default_search_patient in /var/www/openemr/interface/billing/new_payment.php on line 407
Undefined index: mode in /var/www/openemr/interface/billing/new_payment.php on line 409