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_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4d2c7beb696b377a3738e0d4d3f7eed4a2e6d75c

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