Don't set SourceValidate pointer to NULL

inspired from similar code in amdgpu, fixes a crash when xrandr(1)
is invoqued with X server 21.1.1

Signed-off-by: Matthieu Herrb <matthieu.herrb@laas.fr>
12 jobs for xserver-21.1-fixes in 2 minutes and 9 seconds (queued for 18 seconds)
latest
Status Name Job ID Coverage
  Docker Image
passed container-build #15795530

00:00:57

 
  Build
passed build-clang: [1.18] #15795537

00:00:53

passed build-clang: [1.19] #15795538

00:00:57

passed build-clang: [1.20] #15795536

00:00:57

passed build-distcheck #15795531

00:01:11

passed build-gcc: [1.16] #15795532

00:00:51

passed build-gcc: [1.17] #15795533

00:00:54

passed build-gcc: [1.18] #15795534

00:00:51

passed build-gcc: [1.19] #15795535

00:00:52

passed build-noglamor: [1.13] #15795539

00:00:49

passed build-noglamor: [1.14] #15795540

00:00:51

passed build-noglamor: [1.15] #15795541

00:00:53