BUG #15804: Assertion failure when using logging_collector with EXEC_BACKEND

The following bug has been logged on the website:

Bug reference:      15804
Logged by:          Yulian Khodorkovskiy
Email address:      yuli.khodorkovskiy@crunchydata.com
PostgreSQL version: Unsupported/Unknown
Operating system:   Centos 7.4
Description:        

The following assertion fails when compiling postgres 12 on Linux (centos
7.4) with EXEC_BACKEND and logging_collector enabled:

    `Assert(UsedShmemSegAddr != NULL);` in `PGSharedMemoryNoReAttach()`

Commit 57431a911d3a650451d198846ad3194900666152 appears to have introduced
this regression by moving SysLogger_Start() before reset_shared() is called
and shared memory is initialized.

For what it's worth, Windows 10/jacana (and maybe other windows builds) on
the build farm does not use logging_collector, which is perhaps why this
regression was not caught.

Yuli