Further harden nbtree posting split code.
Add more defensive checks around posting list split code. These should
detect corruption involving duplicate table TIDs earlier and more
reliably than any existing check.
Follow up to commit
8f72bbac.
Discussion: https://postgr.es/m/CAH2-WzkrSY_kjyd1_M5xJK1uM0govJXMxPn8JUSvwcUOiHuWVw@mail.gmail.com
Backpatch: 13-, where nbtree deduplication was introduced.