SSL: avoid calling SSL_write() with zero data size. According to documentation, calling SSL_write() with num=0 bytes to be sent results in undefined behavior. We don't currently call ngx_ssl_send_chain() with empty chain and buffer. This check handles the case of a chain with total data size that is a multiple of NGX_SSL_BUFSIZE, and with the special buffer at the end. In practice such cases resulted in premature connection close and critical error "SSL_write() failed (SSL:)" in the error log.