domain names: fix compression of root-label

If the root label is compressed (which is 2 bytes, the root label
itself is only 1 byte, so why do it?), go dns incorrectly set the
offset when encountering such a name.

Fixes miekg#234

msg.go

@@ -414,7 +414,10 @@ Loop:
 			if c == 0x00 {
 				// end of name
 				if len(s) == 0 {
-					return ".", off, nil
+					if ptr == 0 {
+						off1 = off
+					}
+					return ".", off1, nil
 				}
 				break Loop
 			}