darwinssl: Fix bug where packets were sometimes transmitted twice

There was a bug where, if SSLWrite() returned errSSLWouldBlock but did
succeed in transmitting at least something, then we'd incorrectly
resend the packet. Now we never take errSSLWouldBlock as a sign that
nothing was transferred to/from the server.

Bug: http://curl.haxx.se/mail/lib-2013-01/0295.html
Reported by: Bruno de Carvalho