Check syscache result in AlterStatistics

The syscache lookup may return NULL even for valid OID, for example due
to a concurrent DROP STATISTICS, so a HeapTupleIsValid is necessary.
Without it, it may fail with a segfault.

Reported by Alexander Lakhin, patch by me. Backpatch to 13, where ALTER
STATISTICS ... SET STATISTICS was introduced.

Backpatch-through: 13
Discussion: https://postgr.es/m/17372-bf3b6e947e35ae77%40postgresql.org

src/backend/commands/statscmds.c

@@ -677,6 +677,8 @@ AlterStatistics(AlterStatsStmt *stmt)
 	rel = table_open(StatisticExtRelationId, RowExclusiveLock);
 
 	oldtup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(stxoid));
+	if (!HeapTupleIsValid(oldtup))
+		elog(ERROR, "cache lookup failed for extended statistics object %u", stxoid);
 
 	/* Must be owner of the existing statistics object */
 	if (!pg_statistics_object_ownercheck(stxoid, GetUserId()))