tar/asm: don't add a padding entry if it has no bytes

Fixes #65

if the read bytes is 0, then don't even create the entry for that
padding.
This sounds like the solution for the issue opened, but I haven't found
a reproducer for this issue yet. :-\

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

tar/asm/disassemble.go

@@ -135,6 +135,9 @@ func NewInputTarStream(r io.Reader, p storage.Packer, fp storage.FilePutter) (io
 				}
 				isEOF = true
 			}
+			if n == 0 {
+				break
+			}
 			_, err = p.AddEntry(storage.Entry{
 				Type:    storage.SegmentType,
 				Payload: paddingChunk[:n],