win: keep a reference to AFD_POLL_INFO in cancel poll

This fixes the following segfault in pyuv for me: https://github.com/saghul/pyuv/issues/191

@saghul 
Like @saghul said, this would be best done in uv_poll_init().

Minor style issue: libuv prefers `sizeof(*handle->afd_poll_info_2.afd_poll_info_ptr)` rather than `sizeof(AFD_POLL_INFO)`.  The former is a mouthful but it keeps working when the type of `afd_poll_info_ptr` changes.