When working without a current event base, don't try to use IOCP listeners

When working without a current event base, don't try to use IOCP listeners

This fixes a bug turned up with the http unit tests, where we create
the evhttp object using an implicit (NULL) event_base.  This failed
pretty badly when we tried to use IOCP-based listeners.  We could
hunt for the current base from inside listener.c in the future, or
get the iocp base some other way, but for now this is probably the safest
solution.