Make WriteEventsToClient/WriteToClient no-op on fake or dead clients.

This matches the test in TryClientEvents, and is a superset of tests
done by the callers of these functions. The consequence of forgetting
these tests is a server crash, so they're always desirable. In my
opinion, it's better to not require the callers to remember to do these
checks.

For callers that don't do very much work before calling WriteToClient or
WriteEventsToClient, I've removed the redundant checks.

hw/xquartz/xpr/appledri.c has an interesting case: While its check for
"client == NULL" appears redundant with the test in WriteEventsToClient,
it dereferences client to get the sequence number.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=27497
Signed-off-by: Jamey Sharp <jamey@minilop.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>

xfixes/select.c

@@ -78,8 +78,7 @@ XFixesSelectionCallback (CallbackListPtr *callbacks, pointer data, pointer args)
     for (e = selectionEvents; e; e = e->next)
     {
 	if (e->selection == selection->selection && 
-	    (e->eventMask & eventMask) &&
-	    !e->pClient->clientGone)
+	    (e->eventMask & eventMask))
 	{
 	    xXFixesSelectionNotifyEvent	ev;