Thou shalt not corrupt the answer of a cache request

apps/files_sharing/lib/readonlycache.php

@@ -28,7 +28,9 @@ use OC\Files\Cache\Cache;
 class ReadOnlyCache extends Cache {
 	public function get($path) {
 		$data = parent::get($path);
-		$data['permissions'] &= (\OCP\Constants::PERMISSION_READ | \OCP\Constants::PERMISSION_SHARE);
+		if ($data !== false) {
+			$data['permissions'] &= (\OCP\Constants::PERMISSION_READ | \OCP\Constants::PERMISSION_SHARE);
+		}
 		return $data;
 	}