Menus screen enters an infinite loop of loading site's pages

[crazytonyli]

Expected behavior & Actual behavior

When a site does not have any menu, the Menus screen fetches all pages in the site and creates menu items for each top level page. The API requests to load all pages should stop once all pages are loaded. But it doesn't stop. The API request continues to fire.

The infinity loading causes the "default menu" section the Menu screen staying at "loading menus" state.

Steps to reproduce the behavior

To reproduce this bug, prepare a site that doesn't have any menu, like a brand new WP.com site. Add a couple of pages to the site.

1. Adds a log at this line in PostService
2. Launch the app from Xcode and go to the prepared site from the app.
3. Go to the Menu screen (My Site -> More -> Menu)
4. Check out Xcode's console output. The log added at step 1 will keep printing in the console.

Tested on [device], iOS [version], Jetpack iOS / WordPress iOS [version]

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