zebra: stop crash on process termination due to stale ifp->node

zebra: stop crash on process termination due to stale ifp->node

Problem reported that crash occurred when stopping quagga in certain
circumstances.  Determined that this was due to a stale pointer on the
ifp for a deleted interface.  The ifp->node had been freed but the ifp
still kept a pointer to it, and when later the process was stopped, it
attempted to delete it again.

Ticket: CM-15550
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>