vulkan/util: Make multialloc succeed with 0 allocations.

vulkan/util: Make multialloc succeed with 0 allocations.

I wanted to use it for the attachments and clear values of a
vkCmdBeginRenderPass(), but both can be 0 count.  In that case, we would
end up with vk_default_alloc(0,0) because nothing had set the alignment,
and assertion fail instead of allocating 0 bytes.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20994>