Add missed NULL checks for XLogReaderAllocate

XLogReaderAllocate returns NULL if the xlogreader couldn't be
allocated. This NULL checks were forgotten in several places of
twophase.c and caused segmentation faults under heavy workloads.

src/backend/access/transam/twophase.c

@@ -1401,6 +1401,11 @@ FinishPreparedTransaction(const char *gid, bool isCommit, bool raiseErrorIfNotFo
 	 * file descriptor here.
 	 */
 	xlogreader = XLogReaderAllocate(&read_local_xlog_page, NULL);
+	if (!xlogreader)
+		ereport(ERROR,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("out of memory"),
+			 errdetail("Failed while allocating an XLog reading processor.")));
 
 	tfRecord = XLogReadRecord(xlogreader, tfXLogRecPtr, &errormsg);
 	if (tfRecord == NULL)
@@ -1687,6 +1692,11 @@ PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
 	}
 
 	xlogreader = XLogReaderAllocate(&read_local_xlog_page, NULL);
+	if (!xlogreader)
+		ereport(ERROR,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("out of memory"),
+			 errdetail("Failed while allocating an XLog reading processor.")));
 
 	while (tfXLogRecPtr != InvalidXLogRecPtr)
 	{
@@ -1913,6 +1923,11 @@ RecoverPreparedTransactions(void)
 	char	   *errormsg;
 
 	xlogreader = XLogReaderAllocate(&read_local_xlog_page, NULL);
+	if (!xlogreader)
+		ereport(ERROR,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("out of memory"),
+			 errdetail("Failed while allocating an XLog reading processor.")));
 
 	if (crashRecoverPostCheckpointPreparedTransactions_map_ht != NULL)
 	{

src/test/walrep/gplibpq.c

@@ -442,6 +442,11 @@ check_ao_record_present(unsigned char type, char *buf, Size len,
 	test_PrintLog("wal end record", walEnd, sendTime);
 
 	xlogreader = XLogReaderAllocate(&read_local_xlog_page, NULL);
+	if (!xlogreader)
+		ereport(ERROR,
+			(errcode(ERRCODE_OUT_OF_MEMORY),
+			 errmsg("out of memory"),
+			 errdetail("Failed while allocating an XLog reading processor.")));
 
 	/*
 	 * Find the first valid record at or after the given starting point.