Change bufferevent_openssl::do_write so it doesn't call SSL_write wit…

…h a 0 length buffer

I was running into a problem when using bufferevent_openssl with a
very simple echo server.  My server simply bufferevent_read_buffer 'd
data into an evbuffer and then passed that evbuffer straight to
bufferevent_write_buffer.

The problem was every now and again the write would fail for no
apparent reason. I tracked it down to SSL_write being called with the
amount of data to send being 0.

This patch alters do_write in bufferevent_openssl so that it skips
io_vecs with 0 length.

bufferevent_openssl.c

@@ -617,6 +617,12 @@ do_write(struct bufferevent_openssl *bev_ssl, int atmost)
 		if (bev_ssl->bev.write_suspended)
 			break;
 
+		/* SSL_write will (reasonably) return 0 if we tell it to
+		   send 0 data.  Skip this case so we don't interpret the
+		   result as an error */
+		if (space[i].iov_len == 0)
+			continue;
+
 		r = SSL_write(bev_ssl->ssl, space[i].iov_base,
 		    space[i].iov_len);
 		if (r > 0) {