don't update storage mtime if we can't get the modified date

lib/private/Files/Cache/Updater.php

@@ -231,7 +231,10 @@ class Updater implements IUpdater {
 		$parentId = $this->cache->getParentId($internalPath);
 		$parent = dirname($internalPath);
 		if ($parentId != -1) {
-			$this->cache->update($parentId, array('storage_mtime' => $this->storage->filemtime($parent)));
+			$mtime = $this->storage->filemtime($parent);
+			if ($mtime !== false) {
+				$this->cache->update($parentId, array('storage_mtime' => $mtime));
+			}
 		}
 	}
 }