NodeIterator: keep first_item besides first_node to spare wrapper invocations

The NodeIterator is sometimes invocated with node wrappers that are heavy to
invocate, e.g. because they trigger requests. Storing the resulting item
besides the node allows to avoid those extra requests.

In the FrozenNodeIterator, the node is still saved, rather than the item.

instaloader/nodeiterator.py

@@ -99,6 +99,7 @@ class NodeIterator(Iterator[T]):
         else:
             self._data = self._query()
         self._first_node: Optional[Dict] = None
+        self._first_item: Optional[T] = None
         self._is_first = is_first
 
     def _query(self, after: Optional[str] = None) -> Dict:
@@ -151,10 +152,10 @@ class NodeIterator(Iterator[T]):
             item = self._node_wrapper(node)
             if self._is_first is not None:
                 if self._is_first(item, self.first_item):
-                    self._first_node = node
+                    self._first_node, self._first_item = node, item
             else:
                 if self._first_node is None:
-                    self._first_node = node
+                    self._first_node, self._first_item = node, item
             return item
         if self._data.get('page_info', {}).get('has_next_page'):
             query_response = self._query(self._data['page_info']['end_cursor'])
@@ -201,7 +202,15 @@ class NodeIterator(Iterator[T]):
         .. versionchanged:: 4.9.2
            What is considered the first item can be overridden.
         """
-        return self._node_wrapper(self._first_node) if self._first_node is not None else None
+        if self._first_item is None:
+            # Restore _first_item from _first_node. Needed after thawing, since
+            # FrozenNodeIterator stores the node, not the item.
+            self._first_item = (
+                self._node_wrapper(self._first_node)
+                if self._first_node is not None
+                else None
+            )
+        return self._first_item
 
     @staticmethod
     def page_length() -> int: