Fix calculation of RecentGlobalXmin with 64-bit xids assuming there is no wraparound.

Author: Alexander Korotkov

src/backend/storage/ipc/procarray.c

@@ -1798,9 +1798,10 @@ GetSnapshotData(Snapshot snapshot)
 		globalxmin = xmin;
 
 	/* Update global variables too */
-	RecentGlobalXmin = globalxmin - vacuum_defer_cleanup_age;
-	if (!TransactionIdIsNormal(RecentGlobalXmin))
+	if (globalxmin <= FirstNormalTransactionId + vacuum_defer_cleanup_age)
 		RecentGlobalXmin = FirstNormalTransactionId;
+	else
+		RecentGlobalXmin = globalxmin - vacuum_defer_cleanup_age;
 
 	/* Check whether there's a replication slot requiring an older xmin. */
 	if (TransactionIdIsValid(replication_slot_xmin) &&