Fix assertion failure by an immediate shutdown.

In PM_WAIT_DEAD_END state, checkpointer process must be dead already.
But an immediate shutdown could make postmaster's state machine
transition to PM_WAIT_DEAD_END state even if checkpointer process is
still running,  and which caused assertion failure. This bug was introduced
in commit 457d6cf049.

This patch ensures that postmaster's state machine doesn't transition to
PM_WAIT_DEAD_END state in an immediate shutdown while checkpointer
process is running.

src/backend/postmaster/postmaster.c

@@ -3328,19 +3328,21 @@ PostmasterStateMachine(void)
 		 * PM_WAIT_BACKENDS state ends when we have no regular backends
 		 * (including autovac workers), no bgworkers (including unconnected
 		 * ones), and no walwriter, autovac launcher or bgwriter.  If we are
-		 * doing crash recovery then we expect the checkpointer to exit as
+		 * doing crash recovery or an immediate shutdown then we expect
-		 * well, otherwise not. The archiver, stats, and syslogger processes
+		 * the checkpointer to exit as well, otherwise not. The archiver,
-		 * are disregarded since they are not connected to shared memory; we
+		 * stats, and syslogger processes are disregarded since
-		 * also disregard dead_end children here. Walsenders are also
+		 * they are not connected to shared memory; we also disregard
-		 * disregarded, they will be terminated later after writing the
+		 * dead_end children here. Walsenders are also disregarded,
-		 * checkpoint record, like the archiver process.
+		 * they will be terminated later after writing the checkpoint record,
+		 * like the archiver process.
 		 */
 		if (CountChildren(BACKEND_TYPE_NORMAL | BACKEND_TYPE_WORKER) == 0 &&
 			CountUnconnectedWorkers() == 0 &&
 			StartupPID == 0 &&
 			WalReceiverPID == 0 &&
 			BgWriterPID == 0 &&
-			(CheckpointerPID == 0 || !FatalError) &&
+			(CheckpointerPID == 0 ||
+			 (!FatalError && Shutdown < ImmediateShutdown)) &&
 			WalWriterPID == 0 &&
 			AutoVacPID == 0)
 		{