Regression in iterator after #12178 - iterator leaks

[Piotrek Koszuliński]

​http://tests.ckeditor.dev:1030/tests/plugins/list/list#test%20switch%20list%20type%20%28inside%20definition%20list%29

Fails on IE11 and FF.

First bad commit ​git:bf96388. Ticket: #12178.

[Piotrek Koszuliński]

Explanation:

• We start with: <p>foo^<br></p><h1>bar</h1>.
• We find that start of selection is at the end of <p>.
• After range is shrank we try to fix it to avoid #12178, so we set its start at the end of <p>.
• This makes a following range: `<p>foo]<br>[</p><h1>bar</h1>'.
• It's obviously incorrect and iterator leaks from the paragraph.

[Piotrek Koszuliński]

I wasn't able to implement iterator's test that would fail. There must be some special condition which occurs in the list test. So let it be the validator of the patch. Additionally, I implemented additional test in iterator just in case.

Fixed on master with ​git:fc8748b.