pgsql: psql: Fix psql slash option leaks

psql: Fix psql slash option leaks

psql_scan_slash_option() returns a malloc'd string, but \getresults,
\gset in pipeline mode, \restrict, and \unrestrict did not free it
after consuming or copying the value.

Free these option strings after use.

Backpatch to all supported versions. In v17 and older, only \restrict
and \unrestrict are affected, so those branches need only that part of
the fix.

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

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bc18487fe1cad25b36dbe06d33ae3cd046cddfeb

Modified Files
--------------
src/bin/psql/command.c | 6 ++++++
1 file changed, 6 insertions(+)