[Xcb] zeroing out

[Jeremy A. Kolb]

I seem to remember a discussion about zeroing out the reply before sending 
it to the client.  I finally have a decent xv going and my xvinfo 
(xcbxvinfo in xcb-demos) works well except that often times at the end of 
strings there are other characters.  The size for a string returned at the 
end of a reply seems to be the size of the string and the padding until 
the end of the reply.  If you run xcbxvinfo you'll see what I mean.

Jeremy