Fix incorrect WAL description for PREPARE TRANSACTION record.

Since commit 8b1dccd, the PREPARE TRANSACTION WAL record includes
information about dropped statistics entries. However, the WAL resource
manager description function for PREPARE TRANSACTION record failed to
parse this information correctly and always assumed there were
no such entries.

As a result, for example, pg_waldump could not display the dropped
statistics entries stored in PREPARE TRANSACTION records.

The root cause was that ParsePrepareRecord() did not set the number of
statistics entries to drop on commit or abort. These values remained
zero-initialized and were never updated from the parsed record.

This commit fixes the issue by properly setting those values during parsing.
With this fix, pg_waldump can now correctly report dropped statistics
entries in PREPARE TRANSACTION records.

Back-patch to v15, where commit 8b1dccd was introduced.

Author: Daniil Davydov <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Discussion: https://postgr.es/m/CAJDiXgh-6Epb2XiJe4uL0zF-cf0_s_7Lw1TfEHDMLzYjEmfGOw@mail.gmail.com
Backpatch-through: 15
(cherry picked from commit 11efaaffac87e7ba4aedbbb612d2eddc37fabfc4)

Author: MasaoFujii

src/backend/access/rmgrdesc/xactdesc.c

@@ -252,6 +252,8 @@ ParsePrepareRecord(uint8 info, xl_xact_prepare *xlrec, xl_xact_parsed_prepare *p
 	parsed->nsubxacts = xlrec->nsubxacts;
 	parsed->nrels = xlrec->ncommitrels;
 	parsed->nabortrels = xlrec->nabortrels;
+	parsed->nstats = xlrec->ncommitstats;
+	parsed->nabortstats = xlrec->nabortstats;
 	parsed->nmsgs = xlrec->ninvalmsgs;
 
 	strncpy(parsed->twophase_gid, bufptr, xlrec->gidlen);