Initialize shared memory copy of ckptXidEpoch correctly when not in recovery.

This bug was introduced by commit 20d98ab6e4,
so backpatch this to 9.0-9.2 like that one.

This fixes bug #6710, reported by Tarvi Pillessaar

src/backend/access/transam/xlog.c

@@ -6252,11 +6252,14 @@ StartupXLOG(void)
 		ereport(PANIC,
 				(errmsg("invalid next transaction ID")));
 
+	/* initialize shared memory variables from the checkpoint record */
 	ShmemVariableCache->nextXid = checkPoint.nextXid;
 	ShmemVariableCache->nextOid = checkPoint.nextOid;
 	ShmemVariableCache->oidCount = 0;
 	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
 	SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
+	XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
+	XLogCtl->ckptXid = checkPoint.nextXid;
 
 	/*
 	 * We must replay WAL entries using the same TimeLineID they were created
@@ -6340,10 +6343,6 @@ StartupXLOG(void)
 		/* No need to hold ControlFileLock yet, we aren't up far enough */
 		UpdateControlFile();
 
-		/* initialize shared-memory copy of latest checkpoint XID/epoch */
-		XLogCtl->ckptXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
-		XLogCtl->ckptXid = ControlFile->checkPointCopy.nextXid;
-
 		/* initialize our local copy of minRecoveryPoint */
 		minRecoveryPoint = ControlFile->minRecoveryPoint;