Fix nlri parsing of L3VPN prefixes in withdrawals.

Fix nlri parsing of L3VPN prefixes in withdrawals.

L3VPN NLRI have different encoding for updates and withdraws. The withdraw
carries one dummy MPLS label that needs to be skipped. The code doing that
did adjust the lenght but did not skip the the label in the buffer and so
the parsed prefix was off by 3 bytes.
OK tb@