Properly determine length for on-disk TOAST values

Properly determine length for on-disk TOAST values

In detoast_attr_slice, VARSIZE_ANY was used to compute compressed length
of on-disk TOAST values. That's incorrect, because the varlena value may
be just a TOAST pointer, producing either bogus value or crashing.

This is likely why the code was crashing on big-endian machines before
540f31680913 replaced the VARSIZE with VARSIZE_ANY, which however only
masked the issue.

Reported-by: Rushabh Lathia
Discussion: https://postgr.es/m/CAL-OGkthU9Gs7TZchf5OWaL-Gsi=hXqufTxKv9qpNG73d5na_g@mail.gmail.com