pgsql: Fix use-after-free in COPY (ON_ERROR)

Fix use-after-free in COPY (ON_ERROR)

When ON_ERROR handled a soft error, some of the contents set for the
error context callback could remain set and pointing at areas that could
be reset, leading to some use-after-frees depending on the input data
provided through COPY.

These contents are now reset at the beginning of NextCopyFrom(), before
processing the next tuple.

SET_NULL is affected down to v19, IGNORE down to v17.

Author: Gowdham M <gowdham(dot)m(at)zohocorp(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Shihao Zhong <zhong950419(at)gmail(dot)com>
Discussion: https://postgr.es/m/1a181ff6be8.6d14feb261368.4036709598649280477@zohocorp.com
Backpatch-through: 17

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/23e2a332dd24c6b67f4a9bdbb3e0dbdb0a527156

Modified Files
--------------
src/backend/commands/copyfromparse.c | 4 ++++
src/test/regress/expected/copy2.out | 8 ++++++++
src/test/regress/sql/copy2.sql | 11 +++++++++++
3 files changed, 23 insertions(+)