kmswebrtcendpoint: fix memory leak freeing certificate_pem_file attr
kmswebrtcendpoint: fix memory leak freeing certificate_pem_file attr

Change-Id: Ibcbf663d6e47cd1e4eb4e881650c5c08418d1bac

Kurento/kms-elements,Kurento/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,Kurento/kms-elements,TribeMedia/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements