kmsalphablending: Fix race condition accesing to elements in port data when they are deleted
kmsalphablending: Fix race condition accesing to elements in port data when they are deleted

Change-Id: I1caed134751afa55a0496273af2099e08ba79e05

Kurento/kms-elements,Kurento/kms-elements,Kurento/kms-elements