Fix varatt versus Datum type confusions

Fix varatt versus Datum type confusions

Macros like VARDATA() and VARSIZE() should be thought of as taking
values of type pointer to struct varlena or some other related struct.
The way they are implemented, you can pass anything to it and it will
cast it right.  But this is in principle incorrect.  To fix, add the
required DatumGetPointer() calls.  Or in a couple of cases, remove
superfluous PointerGetDatum() calls.

It is planned in a subsequent patch to change macros like VARDATA()
and VARSIZE() to inline functions, which will enforce stricter typing.
This is in preparation for that.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/928ea48f-77c6-417b-897c-621ef16685a6%40eisentraut.org