Fix category id retrieval

[berliozd]

A category instance is passed when a category id is supposed to be received.

Description

In magento2/app/code/Magento/Catalog/Block/Product/ListProduct.php, in _getProductCollection function, the following code set categoryId with a category instance.

// if the product is associated with any category
                if ($categories->count()) {
                    // show products from this category
                    $this->setCategoryId(current($categories->getIterator()));
                }

That causes the following code to fail later :
$category = $this->categoryRepository->get($this->getCategoryId());

Manual testing scenarios

1. In the backoffice, add a product to the upsell product list
2. Go to the frontend product page to verify the bug is occuring
   You should have the following error :

Exception #0 (Exception): Warning: Illegal offset type in /var/www/html/_0521-barbarabui/vendor/magento/module-catalog/Model/CategoryRepository.php on line 133

3. After the fix the bug doesn't occur anymore.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[orlangur]

@berliozd thanks for your contribution!

Seems to be a backport of #7598 (fix present in 2.2), if this test will fail it can be added from that PR.

[vrann]

@berliozd can you please help with the testing of this PR? Here is a video on how it been tested, but issue can't be reproduced.
upsell.zip

[ishakhsuvarov]

Hi @berliozd
Would you still like to proceed with this PR? Please check the comment above if so.
Thanks!

[orlangur]

Although this PR is a carbon copy of https://github.com/magento/magento2/pull/7598/files I don't think merging it without testing is a good idea.

@berliozd, I'm closing this PR for now due to inactivity. Feel free to reach me out anytime later if you manage to find steps to reproduce issue and this PR will be reopened.