gip_send_pkt: Only copy data if non-NULL.

This will, currently, never happen because gip_send_pkt is only called
with NULL data when hdr->packet_length <= GIP_PKT_MAX_LENGTH. It's a
bit weird but gip_send_pkt_simple does it the same way. Not my
prefered solution but works fine for now.

Fixes #28.

Author: dlundqvist

bus/protocol.c

@@ -379,7 +379,8 @@ static int gip_send_pkt(struct gip_client *client,
 	if (err)
 		return err;
 
-	memcpy(client->chunk_buf_in->data, data, hdr->chunk_offset);
+	if (data)
+		memcpy(client->chunk_buf_in->data, data, hdr->chunk_offset);
 
 	return 0;
 }