isisd: refuse adjacencies with our own system ID

isisd: refuse adjacencies with our own system ID

isisd would form an adjacency with another router despite the system IDs
being identical.  This would later cause an assertion failure like this:

  assertion=0x555555596db8 "isis_find_vertex (spftree->paths, id, vtype) == ((void *)0)",
  file=0x555555596c60 "isis_spf.c", line=515, function=0x555555597900 "isis_spf_add2tent") at log.c:619

which is caused by trying to add a path expected to not exist, but
suddenly colliding due to the duplicate system ID.

* isis_pdu.c: check for system ID collision on receiving Hello

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>