validate dangerouse construction (#11873)

Author: ivanmorozov333

ydb/core/tx/columnshard/engines/scheme/indexes/abstract/program.cpp

@@ -443,7 +443,12 @@ class TNormalForm {
 
 std::shared_ptr<TDataForIndexesCheckers> TDataForIndexesCheckers::Build(const TProgramContainer& program) {
     AFL_DEBUG(NKikimrServices::TX_COLUMNSHARD)("program", program.DebugString());
-    auto fStep = program.GetSteps().front();
+    auto& steps = program.GetStepsVerified();
+    if (!steps.size()) {
+        AFL_WARN(NKikimrServices::TX_COLUMNSHARD)("event", "no_steps_in_program");
+        return nullptr;
+    }
+    auto fStep = steps.front();
     TNormalForm nForm;
     for (auto&& s : fStep->GetAssignes()) {
         if (!nForm.Add(s, program)) {

ydb/core/tx/program/program.h

@@ -85,6 +85,11 @@ class TProgramContainer {
         }
     }
 
+    const std::vector<std::shared_ptr<NSsa::TProgramStep>>& GetStepsVerified() const {
+        AFL_VERIFY(!!Program);
+        return Program->Steps;
+    }
+
     template <class TDataContainer>
     inline arrow::Status ApplyProgram(std::shared_ptr<TDataContainer>& batch) const {
         if (Program) {