Make sure we only require what is allowed

[nickvergessen]

Fix nextcloud/server#1708

[MorrisJobke]

@nickvergessen How to reproduce this?

[nickvergessen]

As mentioned in the issue, I have no idea. I just know it works.
Theory casing:

files_accesscontrol/lib/StorageWrapper.php

Lines 392 to 405 in 43d0d12

	/**
	* get a cache instance for the storage
	*
	* @param string $path
	* @param \OC\Files\Storage\Storage (optional) the storage to pass to the cache
	* @return \OC\Files\Cache\Cache
	*/
	public function getCache($path = '', $storage = null) {
	if (!$storage) {
	$storage = $this;
	}
	$cache = $this->storage->getCache($path, $storage);
	return new CacheWrapper($cache, $storage, $this->operation);
	}

untyped storage get's injected into the method, and therefor may not be $this but any storage. so I used the most basic public interface to be sure.

[nickvergessen]

Merging as a second user reports the same issue and says this fixes it.