swift: fix dangling large objects breaking the listing

Before this change, large objects which had had their contents deleted
would return "Object not found" and break the listing.

This change makes these objects appear as 0 sized entities so they can
be listed and deleted.

backend/swift/swift.go

@@ -505,7 +505,15 @@ func (f *Fs) newObjectWithInfo(remote string, info *swift.Object) (fs.Object, er
 	// making sure we read the full metadata for all 0 byte files.
 	// We don't read the metadata for directory marker objects.
 	if info != nil && info.Bytes == 0 && info.ContentType != "application/directory" {
-		info = nil
+		err := o.readMetaData() // reads info and headers, returning an error
+		if err == fs.ErrorObjectNotFound {
+			// We have a dangling large object here so just return the original metadata
+			fs.Errorf(o, "dangling large object with no contents")
+		} else if err != nil {
+			return nil, err
+		} else {
+			return o, nil
+		}
 	}
 	if info != nil {
 		// Set info but not headers