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_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/076b09123078033ebef609c91a9a276ee6e1bcb3

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(-)