compositemixer: Release port data directly if mixer is already released
compositemixer: Release port data directly if mixer is already released

Change-Id: I95529da708938a432bdbe1f983a4710f01e7a072

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