Merge of r5017: fixed null dereference with resolver and poll.

Merge of r5017: fixed null dereference with resolver and poll.

Events: fixed null pointer dereference with resolver and poll.

A POLLERR signalled by poll() without POLLIN/POLLOUT, as seen on
Linux, would generate both read and write events, but there's no
write event handler for resolver events.  A fix is to only call
event handler of an active event.