compositor: Ignore XError when changing background picture

If xfwm4 is compiled with support for the root pixmap in the compositor,
changing the background image may cause an XError if the external pixmap
is already freed.

Guard against such XError.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Closes: #476
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Status Job ID Name Coverage
  Build
passed #8619
build

00:01:07

 
  Distcheck
passed #8620
distcheck

00:01:45