Xi: unvalidated lengths in Xinput extension [CVE-2014-8095]

Multiple functions in the Xinput extension handling of requests from
clients failed to check that the length of the request sent by the
client was large enough to perform all the required operations and
thus could read or write to memory outside the bounds of the request
buffer.

This commit includes the creation of a new REQUEST_AT_LEAST_EXTRA_SIZE
macro in include/dix.h for the common case of needing to ensure a
request is large enough to include both the request itself and a
minimum amount of extra data following the request header.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

Xi/sendexev.c

@@ -135,6 +135,9 @@ ProcXSendExtensionEvent(ClientPtr client)
     if (ret != Success)
         return ret;
 
+    if (stuff->num_events == 0)
+        return ret;
+
     /* The client's event type must be one defined by an extension. */
 
     first = ((xEvent *) &stuff[1]);