lib: Fix nexthop encoding

lib: Fix nexthop encoding

Commit
68a02e06e5f103048d947262c08c569056f74d1c broke nexthop encoding
for nexthop tracking.

This code combined the different types of nexthop encoding
being done in the zapi protocol.  What was missed that
resolved nexthops of type NEXTHOP_TYPE_IPV4|6 have an ifindex
value that was not being reported.  This commit ensures
that we always send this data( even if it is 0).

The following test commit will ensure that this stays working
as is expected by an upper level protocol.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>