pgsql: Fix more portability issues in new amcheck code.

Fix more portability issues in new amcheck code.

verify_heapam() wasn't being careful to sanity-check tuple line
pointers before using them, resulting in SIGBUS on alignment-picky
architectures. Fix that, add some more test coverage.

Mark Dilger, some tweaking by me

Discussion: /messages/by-id/30B8E99A-2D9C-48D4-A55C-741C9D5F1563@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/321633e17b07968e68ca5341429e2c8bbf15c331

Modified Files
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contrib/amcheck/t/001_verify_heapam.pl | 30 ++++++++-----
contrib/amcheck/verify_heapam.c | 78 +++++++++++++++++++++++-----------
2 files changed, 73 insertions(+), 35 deletions(-)