Harden nbtree deduplication posting split code.

Add a defensive "can't happen" error to code that handles nbtree posting
list splits (promote an existing assertion).  This avoids a segfault in
the event of an insertion of a newitem that is somehow identical to an
existing non-pivot tuple in the index.  An nbtree index should never
have two index tuples with identical TIDs.

This scenario is not particular unlikely in the event of any kind of
corruption that leaves the index in an inconsistent state relative to
the heap relation that is indexed.  There are two known reports of
preventable hard crashes.  Doing nothing seems unacceptable given the
general expectation that nbtree will cope reasonably well with corrupt
data.

Discussion: https://postgr.es/m/CAH2-Wz=Jr_d-dOYEEmwz0-ifojVNWho01eAqewfQXgKfoe114w@mail.gmail.com
Backpatch: 13-, where nbtree deduplication was introduced.

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