Do not access indclass through Form_pg_index

Normally, accessing variable-length members of catalog structures past
the first one doesn't work at all.  Here, it happened to work because
indnatts was checked to be 1, and so the defined FormData_pg_index
layout, using int2vector[1] and oidvector[1] for variable-length
arrays, happened to match the actual memory layout.  But it's a very
fragile assumption, and it's not in a performance-critical path, so
code it properly using heap_getattr() instead.

bug analysis by Tom Lane

src/backend/utils/cache/relcache.c

@@ -3351,15 +3351,30 @@ RelationGetIndexList(Relation relation)
 	while (HeapTupleIsValid(htup = systable_getnext(indscan)))
 	{
 		Form_pg_index index = (Form_pg_index) GETSTRUCT(htup);
+		Datum		indclassDatum;
+		oidvector  *indclass;
+		bool		isnull;
 
 		/* Add index's OID to result list in the proper order */
 		result = insert_ordered_oid(result, index->indexrelid);
 
+		/*
+		 * indclass cannot be referenced directly through the C struct, because
+		 * it comes after the variable-width indkey field.  Must extract the
+		 * datum the hard way...
+		 */
+		indclassDatum = heap_getattr(htup,
+									 Anum_pg_index_indclass,
+									 GetPgIndexDescriptor(),
+									 &isnull);
+		Assert(!isnull);
+		indclass = (oidvector *) DatumGetPointer(indclassDatum);
+
 		/* Check to see if it is a unique, non-partial btree index on OID */
 		if (index->indnatts == 1 &&
 			index->indisunique && index->indimmediate &&
 			index->indkey.values[0] == ObjectIdAttributeNumber &&
-			index->indclass.values[0] == OID_BTREE_OPS_OID &&
+			indclass->values[0] == OID_BTREE_OPS_OID &&
 			heap_attisnull(htup, Anum_pg_index_indpred))
 			oidIndex = index->indexrelid;
 	}