Additional fixes of memory alignment in pg_mcv_list code

Additional fixes of memory alignment in pg_mcv_list code

Commit d85e0f366a tried to fix memory alignment issues in serialization
and deserialization of pg_mcv_list values, but it was a few bricks shy.
The arrays of uint16 indexes in serialized items was not aligned, and
the both the values and isnull flags were using the same pointer.

Per investigation by Tom Lane on gaur.