ospfd: Fix json Crash with inactive timer

ospfd: Fix json Crash with inactive timer

When nbr->t_inactivity is not active, and
you do a show json over the neighbor it
will crash ospfd.  Fix the code so it
prints out -1 when the timer is inactive.

Ticket:CM-13835
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>
(cherry picked from commit 78d8fcb9623ab4d9cebf6187a451448e056a84bf)
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>