s3: fix multipart copy - fixes #3778

Before this change multipart copies were giving the error

    Range specified is not valid for source object of size

This was due to an off by one error in the range source introduced in
7b1274e29a "s3: support for multipart copy"

backend/s3/s3.go

@@ -1666,8 +1666,8 @@ func calculateRange(partSize, partIndex, numParts, totalSize int64) string {
 	start := partIndex * partSize
 	var ends string
 	if partIndex == numParts-1 {
-		if totalSize >= 0 {
-			ends = strconv.FormatInt(totalSize, 10)
+		if totalSize >= 1 {
+			ends = strconv.FormatInt(totalSize-1, 10)
 		}
 	} else {
 		ends = strconv.FormatInt(start+partSize-1, 10)