anv: set command buffer to NULL when allocations fail

anv: set command buffer to NULL when allocations fail

The spec section 5.2 says:

   "vkAllocateCommandBuffers can be used to create multiple command
   buffers. If the creation of any of those command buffers fails, the
   implementation must destroy all successfully created command buffer
   objects from this command, set all entries of the pCommandBuffers
   array to VK_NULL_HANDLE and return the error."

Fixes:
   dEQP-VK.api.object_management.alloc_callback_fail_multiple.command_buffer_primary
   dEQP-VK.api.object_management.alloc_callback_fail_multiple.command_buffer_secondary

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "13.0 17.0" <mesa-stable@lists.freedesktop.org>