[yugabyte#8390] Fix NPE Handling for indexed_table_id

Summary:
This simple diff handles nullptr cases for indexed_table_id, which stops an observed
crash-on-load issue that occurs when running with a bad index schema and
'master_ignore_deleted_on_load'.

Test Plan:
Manual Testing
1. set_flag TEST_simulate_crash_after_table_marked_deleting 1
2. cqlsh "CREATE TABLE; CREATE INDEX; DROP TABLE"
3. yb-ctl stop & yb-ctl start.  (2x)

Reviewers: mbautin, rao, bogdan

Reviewed By: bogdan

Subscribers: ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D11660

Author: nspiegelberg

src/yb/master/catalog_manager.cc

@@ -3809,6 +3809,11 @@ Status CatalogManager::BackfillIndex(
     indexed_table = GetTableInfo(indexed_table_id);
   }
 
+  if (indexed_table == nullptr) {
+    return STATUS(InvalidArgument, "Empty indexed table",
+                  index_table_identifier.ShortDebugString());
+  }
+
   // TODO(jason): when ready to use INDEX_PERM_DO_BACKFILL for resuming backfill across master
   // leader changes, replace the following (issue #6218).
 
@@ -3980,7 +3985,8 @@ Status CatalogManager::DeleteTable(
       indexed_table_id = PROTO_GET_INDEXED_TABLE_ID(l->pb);
     }
     scoped_refptr<TableInfo> indexed_table = GetTableInfo(indexed_table_id);
-    const bool is_pg_table = indexed_table->GetTableType() == PGSQL_TABLE_TYPE;
+    const bool is_pg_table = indexed_table != nullptr &&
+                             indexed_table->GetTableType() == PGSQL_TABLE_TYPE;
     bool is_transactional;
     {
       Schema index_schema;

src/yb/master/yql_indexes_vtable.cc

@@ -54,6 +54,10 @@ Result<std::shared_ptr<QLRowBlock>> YQLIndexesVTable::RetrieveData(
     }
 
     scoped_refptr<TableInfo> indexed_table = catalog_manager->GetTableInfo(indexed_table_id);
+    // Skip if the index is invalid (bad schema).
+    if (indexed_table == nullptr) {
+      continue;
+    }
     Schema indexed_schema;
     RETURN_NOT_OK(indexed_table->GetSchema(&indexed_schema));