pgsql: psql: Avoid returning oom_buffer from psql slash option scanner

psql: Avoid returning oom_buffer from psql slash option scanner

psql_scan_slash_option() builds option text in a local PQExpBufferData
and returns the buffer's data pointer to its caller. If either the initial
allocation or a later enlargement failed, that data pointer could be
the static PQExpBuffer OOM buffer rather than malloc-owned storage.
The callers could then eventually pass it to free(), causing undefined
behavior.

Detect a broken option buffer before returning it, report OOM, and return
NULL instead. Also avoid evaluating a backtick substitution when the option
buffer is already broken, since doing so could otherwise touch the static
OOM buffer.

This keeps the existing NULL-return convention for slash options. Callers
are not generally changed to distinguish OOM from no option.

Backpatch to all supported versions.

Reported-by: Junwang Zhao <zhjwpku(at)gmail(dot)com>
Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Junwang Zhao <zhjwpku(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwEh3R3=1tx_a5=fTDJ+ycuwxWMEn6bG_Yt4B5P+hE7AVw@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/789ae09a5f21487083c379727e434b7ba88fab6b

Modified Files
--------------
src/bin/psql/command.c | 2 +-
src/bin/psql/psqlscanslash.l | 24 ++++++++++++++++++++++--
2 files changed, 23 insertions(+), 3 deletions(-)