ossl_send: SSL_write() returning 0 is an error too

We found that in specific cases if the connection is abruptly closed,
the underlying socket is listed in a close_wait state. We continue to
call the curl_multi_perform, curl_mutli_fdset etc. None of these APIs
report the socket closed / connection finished.  Since we have cases
where the multi connection is only used once, this can pose a problem
for us. I've read that if another connection was to come in, curl would
see the socket as bad and attempt to close it at that time -
unfortunately, this does not work for us.

I found that in specific situations, if SSL_write returns 0, curl did
not recognize the socket as closed (or errored out) and did not report
it to the application. I believe we need to change the code slightly, to
check if ssl_write returns 0. If so, treat it as an error - the same as
a negative return code.

For OpenSSL - the ssl_write documentation is here:
http://www.openssl.org/docs/ssl/SSL_write.html