Bug-612: Base PCEP serializers - more defensive code

-check mandatory arguments
-set zeros for missing arguments

Change-Id: I02cf0b4c220846f2c8a9cfe3e9d4bd82bad72fa5
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug-612: Base PCEP serializers - more defensive code"