d3d12: tolerate NULL-pointer inside cbuf

This seems to trigger in rare occations where we run out of memory at
really inconvenient times.

I'm not sure this is really the best fix here, or we should make sure we
don't try to bind such a cbuf in the first place...

But, it fixes a crash, so...
23 jobs for !265 with d3d12-null_cbv in 6 minutes and 47 seconds (queued for 5 seconds)
latest detached
Status Job ID Name Coverage
  Container
passed #3934670
aarch64
arm_build

00:00:23

passed #3934671
aarch64
arm_test

00:00:22

passed #3934672
mesa 1809 shell windows
windows_build_vs2019

00:00:07

passed #3934666
x86_build

00:00:22

passed #3934669
x86_build_old

00:00:23

passed #3934667
x86_test-gl

00:00:28

passed #3934668
x86_test-vk

00:00:23

 
  Meson X86 64
passed #3934676
meson-clang

00:02:29

passed #3934675
meson-classic

00:01:33

passed #3934677
meson-clover

00:04:02

passed #3934678
meson-clover-old-llvm

00:02:36

passed #3934674
meson-gallium

00:02:53

passed #3934673
meson-testing

00:06:24

passed #3934679
meson-vulkan

00:02:36

 
  Scons
passed #3934680
scons-win64

00:04:13

 
  Meson Misc
passed #3934682
aarch64
meson-arm64

00:02:07

passed #3934683
aarch64
meson-arm64-build-test

00:01:43

passed #3934681
aarch64
meson-armhf

00:02:07

passed #3934685
meson-i386

00:03:51

passed #3934688
meson-mingw32-x86_64

00:02:39

passed #3934687
kvm
meson-ppc64el

00:02:36

passed #3934686
kvm
meson-s390x

00:02:56

passed #3934684
mesa 1809 docker windows
meson-windows-vs2019

00:05:03