Fix recent pg_walinspect fpi_length bug.

Commit 0276ae4 taught pg_walinspect's pg_get_wal_record_info()
function to output NULLs rather than empty strings for its record
description and block_ref output parameters.  However, it inadvertently
moved the function call that sets fpi_length until after it was already
set.  As a result, pg_get_wal_record_info() always output spurious
fpi_length values of 0.

Fix by switching the order back (but keep the behavioral change).

Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CAH2-WzkJmgSYkt6-smQ+57SxSmov+EKqFZdSimFewosoL_JKoA@mail.gmail.com

Author: petergeoghegan

contrib/pg_walinspect/pg_walinspect.c

@@ -186,6 +186,7 @@ GetWALRecordInfo(XLogReaderState *record, Datum *values,
 	RmgrData	desc;
 	uint32		fpi_len = 0;
 	StringInfoData rec_desc;
+	StringInfoData rec_blk_ref;
 	int			i = 0;
 
 	desc = GetRmgr(XLogRecGetRmid(record));
@@ -197,6 +198,12 @@ GetWALRecordInfo(XLogReaderState *record, Datum *values,
 	initStringInfo(&rec_desc);
 	desc.rm_desc(&rec_desc, record);
 
+	if (XLogRecHasAnyBlockRefs(record))
+	{
+		initStringInfo(&rec_blk_ref);
+		XLogRecGetBlockRefInfo(record, false, true, &rec_blk_ref, &fpi_len);
+	}
+
 	values[i++] = LSNGetDatum(record->ReadRecPtr);
 	values[i++] = LSNGetDatum(record->EndRecPtr);
 	values[i++] = LSNGetDatum(XLogRecGetPrev(record));
@@ -205,23 +212,15 @@ GetWALRecordInfo(XLogReaderState *record, Datum *values,
 	values[i++] = CStringGetTextDatum(id);
 	values[i++] = UInt32GetDatum(XLogRecGetTotalLen(record));
 	values[i++] = UInt32GetDatum(XLogRecGetDataLen(record));
-
 	values[i++] = UInt32GetDatum(fpi_len);
 
 	if (rec_desc.len > 0)
 		values[i++] = CStringGetTextDatum(rec_desc.data);
 	else
 		nulls[i++] = true;
 
-	/* Block references. */
 	if (XLogRecHasAnyBlockRefs(record))
-	{
-		StringInfoData rec_blk_ref;
-
-		initStringInfo(&rec_blk_ref);
-		XLogRecGetBlockRefInfo(record, false, true, &rec_blk_ref, &fpi_len);
 		values[i++] = CStringGetTextDatum(rec_blk_ref.data);
-	}
 	else
 		nulls[i++] = true;