pgsql: Fix syslogger NULL-pointer-dereference in EXEC_BACKEND

Fix syslogger NULL-pointer-dereference in EXEC_BACKEND

Commit 0c8e082fba8d changed the time at which MyBackendType is assigned,
breaking a careful choreography in syslogger to decide when to write
messages to its own log files. Fix by flipping a boolean at the
(approximate) location where previously MyBackendType was set, instead
of depending on MyBackendType directly.

Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reported-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Euler Taveira <euler(at)eulerto(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/ahP-JT4ZRPyobnLb@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fb23cc7e81db181bfb3dcfed6ad0731a3473d4e1

Modified Files
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src/backend/postmaster/syslogger.c | 19 +++++++++++++++++++
src/backend/utils/error/csvlog.c | 2 +-
src/backend/utils/error/elog.c | 6 +++---
src/backend/utils/error/jsonlog.c | 2 +-
src/include/postmaster/syslogger.h | 1 +
5 files changed, 25 insertions(+), 5 deletions(-)