pgsql: pgbench: fix verbose error message corruption with multiple thre

pgbench: fix verbose error message corruption with multiple threads

When pgbench runs with multiple threads and verbose error reporting is
enabled (--verbose-errors), multiple clients can build verbose error
messages concurrently. Previously, a function-local static
PQExpBuffer was used for these messages, causing the buffer to be
shared across threads. This was not thread-safe and could result in
corrupted or incorrect log output.

Fix this by using a local PQExpBufferData instead of a static buffer.
This keeps verbose error messages correct during concurrent execution.

Backpatch to v15, where this issue was introduced.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Alex Guo <guo(dot)alex(dot)hengchen(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwER1AjGXpkKB9t9820NBhMQ_Ghv7=HsKeodUr3=SZsF4g@mail.gmail.com
Backpatch-through: 15

Branch
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master

Details
-------
https://git.postgresql.org/pg/commitdiff/61f8a85a577fd0ea0bf924a88064187bed31eb48

Modified Files
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src/bin/pgbench/pgbench.c | 27 +++++++++++++--------------
1 file changed, 13 insertions(+), 14 deletions(-)