Description:
------------
When calling offsetUnset for the only element in an ArrayObject and then append one item, the resulting position of an associated ArrayIterator doesn't seem to be well defined.

Steps to reproduce:
- Create an ArrayObject with one item.
- Get an ArrayIterator for the ArrayObject.
- Repeat:
  - Call offsetUnset with the key of the one element in the array.
    Note: Calling offsetUnset on the array or the iterator does not change the 
    behaviour.
  - Append an item to the ArrayObject.
  - check if the iterator is valid

Until php 7.2 the iterator did always point to the appended element.

Test script:
---------------
<?php
$a = new ArrayObject(['item']);
$i = $a->getIterator();

for($c = 0; $c < 100; $c++) {
	echo ($i->valid() ? 'valid' : 'invalid') . PHP_EOL;
	if($i->valid()) {
		$a->offsetUnset($i->key());
	}
	$a->append('item');
}



Expected result:
----------------
The behaviour should at least be consistent. It would be best to have the same behaviour as with php7.2, meaning the test script would always output "valid".

Actual result:
--------------
The behaviour is inconsistent. The test script outputs:
valid
valid
valid
valid
valid
valid
valid
valid
invalid
invalid
invalid
invalid
invalid
invalid
invalid
invalid
valid
invalid
valid
invalid