Filesystem Driver: Fix dir split not finishing cleanly

[the-eater]

I was using Stash as the cache backend for Doctrine, and, being nosy replaced the key hash function with a transparent function (fn($data) => $data) and noticed that the names of the files are not completely finished

which was a bit alarming because this can cause unintentional cache conflicts or poisoning*

After digging around I found

Stash/src/Stash/Driver/FileSystem.php

Lines 292 to 298 in 1a9c755

	for ($i = 0; $i < $this->directorySplit; $i++) {
	$start = $len * $i;
	if ($i == $this->directorySplit) {
	$len = $sLen - $start;
	}
	$path .= substr($value, $start, $len) . DIRECTORY_SEPARATOR;
	}

the $i == $this->directorySplit was supposed to prevent this, however this will always be false, thus I have now replaced it with a bit more robust way of splitting that should prevent this.

and which now works correctly as shown here

*: So this can actually open to cache poisoning attacks, but since the default uses md5 which has an even amount of bytes this is a non-issue unless people use key hash functions with an uneven length return, but these are scarce so ¯\_(ツ)_/¯, an additional "is the key as stored in the item the same as requested" check might be a nice to have, and could prevent this

PS: I am on PHP 8.1 and can't run the outdated phpunit (ok ran my new check by hotfixing it)