[PATCH] Align verify_heapam.c error message offset with test expectations

Hi all,

I think there might be an issue with the error messages in contrib/amcheck/verify_heapam.c.
The test comment in src/bin/pg_amcheck/t/004_verify_heapam.pl (line 715) clearly states:
```
# Tuple at offset 43 is the successor of this one
```
This indicates that for the test case at offnum == 36, the error message should report "offset 43" (the successor), not "offset 36" (the current tuple).
However, when I updated the test expectation from \d+ wildcard to the exact value offset 43, the test fails.
This makes me wonder whether the current code in verify_heapam.c (lines 777, 793, 799) should be using nextoffnum instead of ctx.offnum.
This would also be consistent with similar error messages at lines 746-747 and 753-754, which use nextoffnum when referring to the produced tuple location.

--
Regards,
Man Zeng
www.openhalo.org