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.

src/backend/access/nbtree/nbtdedup.c

@@ -781,7 +781,19 @@ _bt_swap_posting(IndexTuple newitem, IndexTuple oposting, int postingoff)
 
 	nhtids = BTreeTupleGetNPosting(oposting);
 	Assert(_bt_posting_valid(oposting));
-	Assert(postingoff > 0 && postingoff < nhtids);
+
+	/*
+	 * The postingoff argument originated as a _bt_binsrch_posting() return
+	 * value.  It will be 0 in the event of corruption that makes a leaf page
+	 * contain a non-pivot tuple that's somehow identical to newitem (no two
+	 * non-pivot tuples should ever have the same TID).  This has been known
+	 * to happen in the field from time to time.
+	 *
+	 * Perform a basic sanity check to catch this case now.
+	 */
+	if (!(postingoff > 0 && postingoff < nhtids))
+		elog(ERROR, "posting list tuple with %d items cannot be split at offset %d",
+			 nhtids, postingoff);
 
 	/*
 	 * Move item pointers in posting list to make a gap for the new item's