d3d12: Fix memory leaks on error paths in d3d12_context_create.

Fix defect reported by Coverity Scan.

Resource leak (RESOURCE_LEAK)
leaked_storage: Variable ctx going out of scope leaks the storage
it points to.

Fixes: 2ea15cd6 ("d3d12: introduce d3d12 gallium driver")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
37 jobs for !8291 with fix-cid-1470664 in 9 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Sanity
passed #6411913
sanity

00:00:09

 
  Container
manual #6411916
aarch64 manual
arm_build
manual #6411917
manual
arm_test-base
manual #6411918
windows shell 1809 mesa manual
windows_build_vs2019
manual #6411914
manual
x86_build-base
manual #6411915
manual
x86_test-base
 
  Container 2
created #6411926
android_build
created #6411929
arm64_test
created #6411930
armhf_test
created #6411923
i386_build
created #6411919
kernel+rootfs_amd64
created #6411920
aarch64
kernel+rootfs_arm64
created #6411921
aarch64
kernel+rootfs_armhf
created #6411924
ppc64el_build
created #6411925
s390x_build
created #6411922
x86_build
created #6411927
x86_test-gl
created #6411928
x86_test-vk
 
  Meson X86 64
created #6411936
meson-clang
created #6411937
meson-clover
created #6411933
meson-clover-testing
created #6411934
meson-gallium
created #6411935
meson-release
created #6411931
meson-testing
created #6411932
meson-testing-asan
created #6411938
meson-vulkan
 
  Build Misc
created #6411939
meson-android
created #6411941
aarch64
meson-arm64
created #6411942
aarch64
meson-arm64-build-test
created #6411940
aarch64
meson-armhf
created #6411945
meson-i386
created #6411948
meson-mingw32-x86_64
created #6411947
kvm
meson-ppc64el
created #6411946
kvm
meson-s390x
created #6411943
windows docker 1809 mesa
meson-windows-vs2019
created #6411944
allowed to fail
scons-win64
 
  Layered Backends
created #6411949
windows docker 1809 mesa
test-d3d12-windows