dix: Check for NULL spriteInfo in GetPairedDevice

There is a race when reseting the XServer that causes spriteInfo to be
NULL in GetPairedDevice resulting a segfault and subsequent crash. The
problem was noticed when opening a connection, creating master devices,
destroying master devices and closing the connection during testing.

Signed-off-by: Arthur Williams <taaparthur@gmail.com>
3 jobs for master in 6 minutes and 34 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Docker Image
passed #709125
debian-testing

00:00:29

 
  Build And Test
passed #709126
autotools-build-and-test

00:05:50

passed #709127
meson-build-and-test

00:01:55