fix(event): allocate loop->ios in hloop_init

[prestonbrown]

hloop_init leaves loop->ios unallocated, so the first hio_get() allocates it. EventLoopThread::start() returns once the loop is running, before hloop_run() has created the eventfd io, and WebSocketClient::open() calls createsocket() -> hio_get() on the caller's thread. When those two are the first hio_get() on the loop, both allocate the array and one io is lost. The queued startConnect then reaches iowatcher_add_event with loop->ios.ptr[fd] == NULL:

#0 iowatcher_add_event
#1 hio_add
#2 std::function<void(hv::Event*)>::operator()
#3 hv::EventLoop::onCustomEvent
#4 eventfd_read_cb

We hit it about once in 360 runs of a test that starts an EventLoopThread and immediately connects a WebSocketClient. With the array allocated in hloop_init it did not recur in 1800 runs. fds at or past IO_ARRAY_INIT_SIZE still resize lazily.