Make xcb_disconnect(NULL) safe

Make xcb_disconnect(NULL) safe

Code can be simplified if the deallocation functions can always be called in
cleanup code. So if you have some code that does several things that can go
wrong, one of which is xcb_connect(), after this change, the xcb_connection_t*
variable can be initialized to NULL and xcb_disconnect() can always be called on
the connection object.

References: http://lists.freedesktop.org/archives/xcb/2013-September/008659.html

Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Julien Cristau <jcristau@debian.org>