Prevent backgroundScan() from looping if opendir() is failing for the same path

lib/files/cache/scanner.php

@@ -179,9 +179,11 @@ class Scanner {
 	 * walk over any folders that are not fully scanned yet and scan them
 	 */
 	public function backgroundScan() {
-		while (($path = $this->cache->getIncomplete()) !== false) {
+		$lastPath = null;
+		while (($path = $this->cache->getIncomplete()) !== false && $path !== $lastPath) {
 			$this->scan($path);
 			$this->cache->correctFolderSize($path);
+			$lastPath = $path;
 		}
 	}
 }