ossl_recv: check for an OpenSSL error, don't assume

When we recently started to treat a zero return code from SSL_read() as
an error we also got false positives - which primarily looks to be
because the OpenSSL documentation is wrong and a zero return code is not
at all an error case in many situations.

Now ossl_recv() will check with ERR_get_error() to see if there is a
stored error and only then consider it to be a true error if SSL_read()
returned zero.

Bug: http://curl.haxx.se/bug/view.cgi?id=1249
Reported-by: Nach M. S.
Patch-by: Nach M. S.