Ignore self reference object on empty prefix

When a given prefix is empty and we attempt to list its content AWS
returns that the prefix contains one object with key defined as the
prefix with an extra "/" at the end.

e.g.

If we call ListObjects() passing to it an existing but empty prefix,
say "my/empty/prefix", AWS will return that "my/empty/prefix/" is an
object inside "my/empty/prefix" (ListObjectsOutput.Contents).

This extra "/" causes the upload purging process to panic. On normal
circunstances we never find empty prefixes on S3 but users may touch
it.

Signed-off-by: Ricardo Maraschini <rmarasch@redhat.com>

registry/storage/driver/s3-aws/s3.go

@@ -986,6 +986,11 @@ func (d *driver) doWalk(parentCtx context.Context, objectCount *int64, path, pre
 		}
 
 		for _, file := range objects.Contents {
+			// empty prefixes are listed as objects inside its own prefix.
+			// https://docs.aws.amazon.com/AmazonS3/latest/user-guide/using-folders.html
+			if strings.HasSuffix(*file.Key, "/") {
+				continue
+			}
 			walkInfos = append(walkInfos, walkInfoContainer{
 				FileInfoFields: storagedriver.FileInfoFields{
 					IsDir:   false,