Bug 2492: LinkstateAdjRIBsIn.removeRoutes accepts null routes.

http://tools.ietf.org/html/rfc4760#section-4
does not explicitly forbid MP_UNREACH_NLRI without any
Withdrawn Routes in it, so if statement is added
to handle withdrawnRoutes == null case.
(Patch compiles, but misses a unit test.)

Change-Id: Ie880f9c81d6a3773e4b33dea164d8bab3b6ee248
Signed-off-by: Vratko Polak <vrpolak@cisco.com>