If the received MP nexthop is a martian address, treat the update as

If the received MP nexthop is a martian address, treat the update as
an implicit withdraw as is done for the NEXT_HOP attribute in the
update itself.

Note: Check is implemented only for IPv6 for the global nexthop. The
code will quietly ignore an invalid IPv6 link-local nexthop, if present;
this is the existing behavior and is not changed.

Signed-off-by: Vivek Venkataraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>