Fix interruped downloads with --latest-stamps (#1219)

The most recent post is cached in NodeIterator (and
saved to the disk), and its timestamp is used, instead of the
timestamp instaloader was run.

This way, even in later resuming runs the timestamp stored is the same
that would have been stored if the first run.

Fixes #1206.

instaloader/structures.py

@@ -1643,6 +1643,8 @@ def load_structure(context: InstaloaderContext, json_structure: dict) -> JsonExp
         elif node_type == "Hashtag":
             return Hashtag(context, json_structure['node'])
         elif node_type == "FrozenNodeIterator":
+            if not 'first_node' in json_structure['node']:
+                json_structure['node']['first_node'] = None
             return FrozenNodeIterator(**json_structure['node'])
     elif 'shortcode' in json_structure:
         # Post JSON created with Instaloader v3