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.

 Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=27497Signed-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>
 Backport to nx-libs: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>