multi: CURLINFO_LASTSOCKET doesn't work after remove_handle

When curl_multi_remove_handle() is called and an easy handle is returned
to the connection cache held in the multi handle, then we cannot allow
CURLINFO_LASTSOCKET to extract it since that will more or less encourage
that the user uses the socket while it can get used by libcurl again.

Without this fix, we'd get a segfault in Curl_getconnectinfo() trying to
dereference the NULL pointer in 'data->state.connc'.

Bug: http://curl.haxx.se/bug/view.cgi?id=3023840