pgsql: pgbench: fix misprocessing of some nested \if constructs.

pgbench: fix misprocessing of some nested \if constructs.

An \if command appearing within a false (not-to-be-executed) \if
branch was incorrectly treated the same as \elif. This could allow
statements within the inner \if to be executed when they should
not be. Also the missing inner \if stack entry would result in an
assertion failure (in assert-enabled builds) when the final \endif
is reached.

Report and patch by Michail Nikolaev. Back-patch to all
supported branches.

Discussion: https://postgr.es/m/CANtu0oiA1ke=SP6tauhNqkUdv5QFsJtS1p=aOOf_iU+EhyKkjQ@mail.gmail.com

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ec0dc2c596ff7c950ea3d47bf0994ef0e322ceb7

Modified Files
--------------
src/bin/pgbench/pgbench.c | 16 +++++----
src/bin/pgbench/t/001_pgbench_with_server.pl | 50 ++++++++++++++++++++++++++++
2 files changed, 59 insertions(+), 7 deletions(-)