linux-cp: make check of message ts null-tolerant

linux-cp: make check of message ts null-tolerant

Type: improvement

For some message types, timestamps are checked on netlink message to
decide whether the message should be applied. For notification messages
timestamps are expected to be always available.

With this change, before accessing the timestamp, make sure the message
info object that carries it is not null. If it is null, pass the check.
This is to be ready to process dump replies that will not need the
timestamp check and will have the message info object set to null.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: Ic7211c0d451d72f6a5248898b3a8f8e0bca8f7aa