Clear padding of PgStat_HashKey when handling pgstats entries

PgStat_HashKey is currently initialized in a way that could result in
random data if the structure has any padding bytes.  The structure
has no padding bytes currently, fortunately, but it could become a
problem should the structure change at some point in the future.

The code is changed to use some memset(0) so as any padding would be
handled properly, as it would be surprising to see random failures in
the pgstats entry lookups.  PgStat_HashKey is a structure internal to
pgstats, and an ABI change could be possible in the scope of a bug fix,
so backpatch down to 15 where this has been introduced.

Author: Bertrand Drouvot
Reviewed-by: Jelte Fennema-Nio, Michael Paquier
Discussion: https://postgr.es/m/Zyb7RW1y9dVfO0UH@ip-10-97-1-34.eu-west-3.compute.internal
Backpatch-through: 15

src/backend/utils/activity/pgstat.c

@@ -938,6 +938,9 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
 
 	pgstat_prep_snapshot();
 
+	/* clear padding */
+	memset(&key, 0, sizeof(struct PgStat_HashKey));
+
 	key.kind = kind;
 	key.dboid = dboid;
 	key.objid = objid;

src/backend/utils/activity/pgstat_shmem.c

@@ -432,11 +432,18 @@ PgStat_EntryRef *
 pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, uint64 objid, bool create,
 					 bool *created_entry)
 {
-	PgStat_HashKey key = {.kind = kind,.dboid = dboid,.objid = objid};
+	PgStat_HashKey key;
 	PgStatShared_HashEntry *shhashent;
 	PgStatShared_Common *shheader = NULL;
 	PgStat_EntryRef *entry_ref;
 
+	/* clear padding */
+	memset(&key, 0, sizeof(struct PgStat_HashKey));
+
+	key.kind = kind;
+	key.dboid = dboid;
+	key.objid = objid;
+
 	/*
 	 * passing in created_entry only makes sense if we possibly could create
 	 * entry.
@@ -908,10 +915,17 @@ pgstat_drop_database_and_contents(Oid dboid)
 bool
 pgstat_drop_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
 {
-	PgStat_HashKey key = {.kind = kind,.dboid = dboid,.objid = objid};
+	PgStat_HashKey key;
 	PgStatShared_HashEntry *shent;
 	bool		freed = true;
 
+	/* clear padding */
+	memset(&key, 0, sizeof(struct PgStat_HashKey));
+
+	key.kind = kind;
+	key.dboid = dboid;
+	key.objid = objid;
+
 	/* delete local reference */
 	if (pgStatEntryRefHash)
 	{