singleipconnect: return OK even when Curl_socket() fails

Commit 9109cdec brought this regression (shipped since 7.24.0).

The singleipconnect() function must not return an error if Curl_socket()
returns an error. It should then simply return OK and pass a SOCKET_BAD
back simply because that is how the user of this function expects it to
work and something else is not fine.

Reported by: Blaise Potard
Bug: http://curl.haxx.se/bug/view.cgi?id=3516508