Document issue with heapam line pointer truncation.

Checking that an offset number isn't past the end of a heap page's line
pointer array was just a defensive sanity check for HOT-chain traversal
code before commit 3c3b8a4b.  It's etrictly necessary now, though.  Add
comments that reference the issue to code in heapam that needs to get it
right.

Per suggestion from Alexander Lakhin.

Discussion: https://postgr.es/m/f76a292c-9170-1aef-91a0-59d9443b99a3@gmail.com

src/backend/access/heap/heapam.c

@@ -7483,8 +7483,15 @@ heap_index_delete_tuples(Relation rel, TM_IndexDeleteOp *delstate)
 			ItemId		lp;
 			HeapTupleHeader htup;
 
-			/* Some sanity checks */
-			if (offnum < FirstOffsetNumber || offnum > maxoff)
+			/* Sanity check (pure paranoia) */
+			if (offnum < FirstOffsetNumber)
+				break;
+
+			/*
+			 * An offset past the end of page's line pointer array is possible
+			 * when the array was truncated
+			 */
+			if (offnum > maxoff)
 				break;
 
 			lp = PageGetItemId(page, offnum);

src/backend/access/heap/pruneheap.c

@@ -581,8 +581,15 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate)
 		bool		tupdead,
 					recent_dead;
 
-		/* Some sanity checks */
-		if (offnum < FirstOffsetNumber || offnum > maxoff)
+		/* Sanity check (pure paranoia) */
+		if (offnum < FirstOffsetNumber)
+			break;
+
+		/*
+		 * An offset past the end of page's line pointer array is possible
+		 * when the array was truncated (original item must have been unused)
+		 */
+		if (offnum > maxoff)
 			break;
 
 		/* If item is already processed, stop --- it must not be same chain */
@@ -962,8 +969,15 @@ heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
 		 */
 		for (;;)
 		{
-			/* Sanity check */
-			if (nextoffnum < FirstOffsetNumber || nextoffnum > maxoff)
+			/* Sanity check (pure paranoia) */
+			if (offnum < FirstOffsetNumber)
+				break;
+
+			/*
+			 * An offset past the end of page's line pointer array is possible
+			 * when the array was truncated
+			 */
+			if (offnum > maxoff)
 				break;
 
 			lp = PageGetItemId(page, nextoffnum);