s3: fix corrupted on transfer: sizes differ 0 vs xxxx with Ceph

In this commit, released in 1.56.0 we started reading the size of the
object from the Content-Length header as returned by the GET request
to read the object.

4401d180aa s3: add --s3-no-head-object

However some object storage systems, notably Ceph, don't return a
Content-Length header.

The new code correctly calls the setMetaData function with a nil
pointer to the ContentLength.

However due to this commit from 2014, released in v1.18, the
setMetaData function was not ignoring the size as it should have done.

0da6f24221  s3: use official github.com/aws/aws-sdk-go including multipart upload #101

This commit correctly ignores the content length if not set.

Fixes #5732

backend/s3/s3.go

@@ -2892,14 +2892,12 @@ func (o *Object) readMetaData(ctx context.Context) (err error) {
 }
 
 func (o *Object) setMetaData(etag *string, contentLength *int64, lastModified *time.Time, meta map[string]*string, mimeType *string, storageClass *string) {
-	var size int64
 	// Ignore missing Content-Length assuming it is 0
 	// Some versions of ceph do this due their apache proxies
 	if contentLength != nil {
-		size = *contentLength
+		o.bytes = *contentLength
 	}
 	o.setMD5FromEtag(aws.StringValue(etag))
-	o.bytes = size
 	o.meta = meta
 	if o.meta == nil {
 		o.meta = map[string]*string{}