isisd: check vertex type before checking its data

isisd: check vertex type before checking its data

vertex->N is an union whose "id" and "ip" fields are only valid
depending on the vertex type (IS adjacency or IP reachability
information). As such, add a vertex type check before consulting
vertex->N.id in order to prevent unexpected behavior from happening.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>