radv: Fix descriptor set allocation failure.

Set all the handles to VK_NULL_HANDLE:

"If the creation of any of those descriptor sets fails, then the implementation
must destroy all successfully created descriptor set objects from this command,
set all entries of the pDescriptorSets array to VK_NULL_HANDLE and return the
error."

(Vulkan 1.1.117 Spec, section 13.2)

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 2b53c49d)