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: https://postgr.es/m/30B8E99A-2D9C-48D4-A55C-741C9D5F1563@enterprisedb.com