lib: Prevent infinite loop in fd handling

lib: Prevent infinite loop in fd handling

If we have a case where have created a fd for i/o and we have
removed the handling thread but still have the fd in the poll
data structure, there existed a case where we would get
the handle this fd return from poll but we would immediately
do nothing with it because we didn't have a thread to hand
the event to.

This leads to an infinite loop.  Prevent the infinite loop
from happening and log the problem.

We still need to find the cause of this happening.  But
let's prevent the system from melting down in the mean time.

Fixes: #2796
Signed-off-by: David Lamparter <equinox@diac24.net>
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>