lavapipe: do not early return for mem alloc size being zero

Per spec VUID-VkMemoryAllocateInfo-allocationSize-07897

> If the parameters do not define an import or export operation,
  allocationSize must be greater than 0.

So early return there is simply messing up with external memory.

Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36252>
1 file changed