Bug in heap_get_root_tuples

Commit c7aeb775df895db240dcd6f47242f7e08899adfb contains the following
change:

-                       /* 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);

So it checks `offnum` and uses `nextoffnum`.