Fix copy-paste error in test_ginpostinglist

Fix copy-paste error in test_ginpostinglist

The check for a mismatch on the second decoded item pointer
was an exact copy of the first item pointer check, comparing
orig_itemptrs[0] with decoded_itemptrs[0] instead of orig_itemptrs[1]
with decoded_itemptrs[1].  The error message also reported (0, 1) as
the expected value instead of (blk, off).  As a result, any decoding
error in the second item pointer (where the varbyte delta encoding
is exercised) would go undetected.

This has been wrong since commit bde7493d1, so backpatch to all
supported versions.

Author: Jianghua Yang <yjhjstz@gmail.com>
Discussion: https://postgr.es/m/CAAZLFmSOD8R7tZjRLZsmpKtJLoqjgawAaM-Pne1j8B_Q2aQK8w@mail.gmail.com
Backpatch-through: 14