pgsql: 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.

pgsql: 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.