CRM-13313 - Payments - prevent fatal error when line-item has no price set

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* CRM-13313: "Pay Later" for Drupal Webforms
  http://issues.civicrm.org/jira/browse/CRM-13313
Merge pull request #1650 from colemanw/master

CRM-13313 - Payments - prevent fatal error when line-item has no price set