Index read fixes (percona#1292)

Upstream commit ID: facebook/mysql-5.6@4c5fa16
PS-8951: Merge percona-202305 (https://jira.percona.com/browse/PS-8951)

Summary:
Minor index access cleanups for MyRocks:
- `ha_rocksdb::index_init`: remove duplicate `active_index` assignment. Move
  `m_need_build_decoder` assignment past the error exit so that taking it does not
  result in any state changes.
- `ha_rocksdb::index_next_same`: add an assert for `active_index` value to prevent
  out-of-bound accesses in the next line.

Pull Request resolved: facebook/mysql-5.6#1292

Reviewed By: sunshine-Chun

Differential Revision: D45277614

Pulled By: hermanlee

fbshipit-source-id: 8624b6e99a85065f6fda2da126c7935294baebee

Author: laurynas-biveinis

storage/rocksdb/ha_rocksdb.cc

@@ -9928,6 +9928,7 @@ int ha_rocksdb::index_next_same(uchar *const buf,
 
   if (m_full_key_lookup) {
 #ifndef NDEBUG
+    assert(active_index != MAX_KEY);
     uint len = calculate_key_len(table, active_index, HA_WHOLE_KEY);
     assert(len == keylen);
 #endif
@@ -11472,9 +11473,6 @@ void ha_rocksdb::check_build_decoder() {
 int ha_rocksdb::index_init(uint idx, bool sorted) {
   DBUG_ENTER_FUNC();
 
-  m_need_build_decoder = true;
-  active_index = idx;
-
   THD *thd = ha_thd();
   if (thd && thd->killed) {
     DBUG_RETURN(HA_ERR_QUERY_INTERRUPTED);
@@ -11483,6 +11481,8 @@ int ha_rocksdb::index_init(uint idx, bool sorted) {
   Rdb_transaction *const tx = get_or_create_tx(thd);
   assert(tx != nullptr);
 
+  m_need_build_decoder = true;
+
   active_index = idx;
   if (idx != table->s->primary_key &&
       m_key_descr_arr[idx]->is_partial_index()) {