Description:
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once valid() becomes false, previous() stops working

https://3v4l.org/A125P

Test script:
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<?php

$iteratorx = IntlCodePointBreakIterator::createCodePointInstance();
$iteratorx->setText("abc");

$iterator = $iteratorx->getPartsIterator();
$iterator->rewind();
$iterator->next();
$iteratorx->previous(); // previous works
$iterator->next();
$iterator->next();
echo $iterator->current() . "\n"; // c... all fine

$iterator->next(); // no more code points, so it's invalid
var_dump($iterator->valid()); // false
var_dump($iterator->current()); // prints weird stuff... whatever...

$iteratorx->previous(); // cannot go back
var_dump($iterator->valid()); // broken
var_dump($iterator->current()); // broken

Expected result:
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I expect valid() to be true after previous(), and current() set to the last code point