XCB: Handle connection errors

XCB: Handle connection errors

When the XCB connection breaks, all following XCB function calls will fail and
function calls that are currently in progress will return. This means that
xcb_wait_for_event() will return NULL and thus the QXcbEventReader thread will
exit.

This patch uses the above behavior to make sure that processXcbEvents() will be
called. The error handling should always be done before normal event processing,
because all XCB calls will fail once the connection is in an error state. This
is especially unexpected for xcb_get_setup() which suddenly returns a NULL
pointer.

Task-number: QTBUG-27686

Change-Id: Ie3e4058f9d92bcbfc45934a8b36d9a7254e2b4bb
Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>