multi: inform multi about closed sockets before they are closed

When the connection code decides to close a socket it informs
the multi system via the Curl_multi_closed function. The multi
system may, in turn, invoke the CURLMOPT_SOCKETFUNCTION function
with CURL_POLL_REMOVE. This happens after the socket has already
been closed. Reorder the code so that CURL_POLL_REMOVE is called
before the socket is closed.