use strict equals in readdir loops to prevent issues with '0' files

Conflicts:
	apps/files_external/lib/amazons3.php
	apps/files_trashbin/index.php
	lib/connector/sabre/objecttree.php
additional readdir check in mappedlocal
Merge pull request #4541 from owncloud/backport-4512-stable5

Backport 4512 stable5