amcheck: Fix incorrect use of VARATT_IS_COMPRESSED.
The macro is being applied to a TOAST pointer, not a varlena header.
Therefore the use of VARATT_IS_COMPRESSED() is wrong. We can check
VARATT_EXTERNAL_IS_COMPRESSED(), but then we don't need the length
check that follows.
Report and fix by Kyotaro Horiguchi.
Discussion: http://postgr.es/m/
20220517.162719.
1671558681467343711.horikyota.ntt@gmail.com