Xi: Zero target buffer in SProcXSendExtensionEvent.

Make sure that the xEvent eventT is initialized with zeros, the same way as
in SProcSendEvent.

Some event swapping functions do not overwrite all 32 bytes of xEvent
structure, for example XSecurityAuthorizationRevoked. Two cooperating
clients, one swapped and the other not, can send
XSecurityAuthorizationRevoked event to each other to retrieve old stack data
from X server. This can be potentialy misused to go around ASLR or
stack-protector.

Signed-off-by: Michal Srb <msrb@suse.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 05442de9)