Do not try to call remove() if filepath is NULL

Change-Id: Ie6635d8a43577792945c823de4cb8a368a7c9142
Reviewed-on: http://review.couchbase.org/33432
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>