[debug] load correct stack slot for frame details.

test/mjsunit/regress/regress-5071.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug
+
+var Debug = debug.Debug;
+
+function listener(event, exec_state, event_data, data) {
+  assertEquals(2, exec_state.frameCount());
+  assertEquals("a", exec_state.frame(0).localName(0));
+  assertEquals("1", exec_state.frame(0).localValue(0).value());
+  assertEquals(1, exec_state.frame(0).localCount());
+}
+
+Debug.setListener(listener);
+
+function f() {
+  var a = 1;
+  {
+    let b = 2;
+    debugger;
+  }
+}
+
+f();