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@kurilemu.de>
Reported-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/ahP-JT4ZRPyobnLb@paquier.xyz