Initialize index stats during parallel VACUUM.

Initialize shared memory allocated for index stats to avoid a hard
crash.  This was possible when parallel VACUUM became confused about the
current phase of index processing.

Oversight in commit 8e1fae1938, which refactored parallel VACUUM.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reported-By: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20220818133406.GL26426@telsasoft.com
Backpatch: 15-, the first version with the refactoring commit.

src/backend/commands/vacuumparallel.c

@@ -317,6 +317,7 @@ parallel_vacuum_init(Relation rel, Relation *indrels, int nindexes,
 
 	/* Prepare index vacuum stats */
 	indstats = (PVIndStats *) shm_toc_allocate(pcxt->toc, est_indstats_len);
+	MemSet(indstats, 0, est_indstats_len);
 	for (int i = 0; i < nindexes; i++)
 	{
 		Relation	indrel = indrels[i];