Xext/shm: Add a sanity check before calling next CloseScreen()

It might be the last CloseScreen(), so let's add a sanity check before
calling the next.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
3 jobs for leak in 6 minutes and 18 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-testing #745042

00:01:47

 
  Build And Test
passed autotools-build-and-test #745043

00:04:30

passed meson-build-and-test #745044

00:01:35