Don't send "cannot connect" errors on invalid startup packet

Commit 16671ba6e7 moved the code that sends "sorry, too many clients
already" and other such messages, but it had the effect that we would
send that error even if the the startup packet processing failed, e.g.
because the client sent an invalid startup packet. That was not
intentional.

Spotted while reading the code again.

src/backend/postmaster/postmaster.c

@@ -4354,42 +4354,45 @@ BackendInitialize(Port *port)
 	 * now instead of wasting cycles on an authentication exchange. (This also
 	 * allows a pg_ping utility to be written.)
 	 */
-	switch (port->canAcceptConnections)
+	if (status == STATUS_OK)
 	{
-		case CAC_STARTUP:
-			ereport(FATAL,
-					(errcode(ERRCODE_CANNOT_CONNECT_NOW),
-					 errmsg("the database system is starting up")));
-			break;
-		case CAC_NOTCONSISTENT:
-			if (EnableHotStandby)
+		switch (port->canAcceptConnections)
+		{
+			case CAC_STARTUP:
 				ereport(FATAL,
 						(errcode(ERRCODE_CANNOT_CONNECT_NOW),
-						 errmsg("the database system is not yet accepting connections"),
-						 errdetail("Consistent recovery state has not been yet reached.")));
-			else
+						 errmsg("the database system is starting up")));
+				break;
+			case CAC_NOTCONSISTENT:
+				if (EnableHotStandby)
+					ereport(FATAL,
+							(errcode(ERRCODE_CANNOT_CONNECT_NOW),
+							 errmsg("the database system is not yet accepting connections"),
+							 errdetail("Consistent recovery state has not been yet reached.")));
+				else
+					ereport(FATAL,
+							(errcode(ERRCODE_CANNOT_CONNECT_NOW),
+							 errmsg("the database system is not accepting connections"),
+							 errdetail("Hot standby mode is disabled.")));
+				break;
+			case CAC_SHUTDOWN:
 				ereport(FATAL,
 						(errcode(ERRCODE_CANNOT_CONNECT_NOW),
-						 errmsg("the database system is not accepting connections"),
-						 errdetail("Hot standby mode is disabled.")));
-			break;
-		case CAC_SHUTDOWN:
-			ereport(FATAL,
-					(errcode(ERRCODE_CANNOT_CONNECT_NOW),
-					 errmsg("the database system is shutting down")));
-			break;
-		case CAC_RECOVERY:
-			ereport(FATAL,
-					(errcode(ERRCODE_CANNOT_CONNECT_NOW),
-					 errmsg("the database system is in recovery mode")));
-			break;
-		case CAC_TOOMANY:
-			ereport(FATAL,
-					(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
-					 errmsg("sorry, too many clients already")));
-			break;
-		case CAC_OK:
-			break;
+						 errmsg("the database system is shutting down")));
+				break;
+			case CAC_RECOVERY:
+				ereport(FATAL,
+						(errcode(ERRCODE_CANNOT_CONNECT_NOW),
+						 errmsg("the database system is in recovery mode")));
+				break;
+			case CAC_TOOMANY:
+				ereport(FATAL,
+						(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
+						 errmsg("sorry, too many clients already")));
+				break;
+			case CAC_OK:
+				break;
+		}
 	}
 
 	/*