Fix assertion failure with latch wait in single-user mode

Fix assertion failure with latch wait in single-user mode

LatchWaitSetPostmasterDeathPos, the latch event position for the
postmaster death event, is initialized under IsUnderPostmaster.
WaitLatch() considered it as a valid wait target in single-user mode
(!IsUnderPostmaster), which was incorrect.

One code path found to fail with an assertion failure is a database drop
in single-user mode while waiting in WaitForProcSignalBarrier() after
the drop.

Oversight in commit 84e5b2f07a5e.

Author: Patrick Stählin <me@packi.ch>
Co-authored-by: Ronan Dunklau <ronan.dunklau@aiven.io>
Discussion: https://postgr.es/m/18996-3a2744c8140488de@postgresql.org
Backpatch-through: 18