PagedEntityContainer: fix advance_page_index() for expired pages

This prevents accessing invalid indices when an iterator is already
pointing within an expired page. This may happen, for example,
in the following case:

1. Get an iterator into page `P`, which has one valid element
   anywhere except the last valid index.
2. Call `erase(it)` for the parent container.
3. Invoke `++it`.

So, previously, the iterator won't notice that the page has expired
after `erase(it)` and will try to just increase the infra-index
inside page `P`, although this will cause access to already released
memory (because, pages release their storage upon expiration).

Fix that by always checking for page expiration first in the `advance_page_index()`.

Signed-off-by: Pavel Solodovnikov <[email protected]>

lib/framework/paged_entity_container.h

@@ -577,8 +577,8 @@ class PagedEntityContainer
 			const auto pagesCount = _c._pages.size();
 			assert(_pageIdx.first < pagesCount);
 			assert(_pageIdx.second <= current_page().max_valid_index());
-			// Move to the next page if already pointing to the last valid index.
-			if (_pageIdx.second == current_page().max_valid_index())
+			// Move to the next page if already pointing to the last valid index or if the current page is expired.
+			if (_pageIdx.second == current_page().max_valid_index() || current_page().is_expired())
 			{
 				// Skip expired pages when transitioning to the next page.
 				do