lib: Correctly handle ppoll pfds.events == 0

lib: Correctly handle ppoll pfds.events == 0

The frrevent system is spitting out this message in bgpd:
20:40:15 mem1-roc-f2-b1-r5-t2-d4 bgpd[13166]: [XETTR-D5MR0][EC 100663316] Attempting to process an I/O event but for fd: 214(8) no thread to handle this!

This is because as each io event is processed, it is possible that a
.events is set to 0.  This can leave a situation where we ask
ppoll to handle anything that happens on a fd with a .events of 0,
in this situation ppoll can return POLLERR, which indicates that
something bad has happened on the fd.

Let's set the ppoll fds.fd value to -1 when there are no more
events to be processed.  ppoll specifically calls out that
it will just skip this particular one.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>