xl: GetCheckSumInfo() shouldn't fail if hash not available. (#2984)

In a multipart upload scenario disks going down and coming backup
can lead to certain parts missing on the disk/server which was
going down. This is a valid case since these blocks can be
missing and should be healed through heal operation. But we are
not supposed to fail prematurely since we have enough data on
the other disks as well within read-quorum.

This fix relaxes previous assumption, fixes a major corruption
issue reproduced by @vadmeste.

Fixes #2976

cmd/xl-v1-multipart.go

@@ -386,8 +386,6 @@ func (xl xlObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 	tmpSuffix := getUUID()
 	tmpPartPath := path.Join(tmpMetaPrefix, tmpSuffix)
 
-	lreader := data
-
 	// Initialize md5 writer.
 	md5Writer := md5.New()
 
@@ -401,12 +399,16 @@ func (xl xlObjects) PutObjectPart(bucket, object, uploadID string, partID int, s
 
 	mw := io.MultiWriter(writers...)
 
+	var lreader io.Reader
 	// Limit the reader to its provided size > 0.
 	if size > 0 {
 		// This is done so that we can avoid erroneous clients sending
 		// more data than the set content size.
 		lreader = io.LimitReader(data, size)
-	} // else we read till EOF.
+	} else {
+		// else we read till EOF.
+		lreader = data
+	}
 
 	// Construct a tee reader for md5sum.
 	teeReader := io.TeeReader(lreader, mw)