select: with winsock, avoid passing unsupported arguments to select()

"Any two of the parameters, readfds, writefds, or exceptfds, can be
given as null. At least one must be non-null, and any non-null
descriptor set must contain at least one handle to a socket."

http://msdn.microsoft.com/en-ca/library/windows/desktop/ms740141(v=vs.85).aspx

When using select(), cURL doesn't adhere to this (WinSock-specific)
rule, and can ask to monitor empty fd_sets, which leads to select()
returning WSAEINVAL (i.e. EINVAL) and connections failing in mysterious
ways as a result (at least when using the curl_multi_socket_action()
interface).

Bug: http://curl.haxx.se/mail/lib-2014-05/0278.html