UnpackDomainName: fix infinite loop where it would lower 'off'

When a pointer points to a empty name, the "return '.'" special case used to
kick in which is not pointer-aware so it would reset the parsing offset to
the pointer target

This was independently found and fixed in c13d4ee, I'm submitting this patch
anyway as it seems a bit more robust and DRY [citation needed].

msg.go

@@ -413,12 +413,6 @@ Loop:
 		case 0x00:
 			if c == 0x00 {
 				// end of name
-				if len(s) == 0 {
-					if ptr == 0 {
-						off1 = off
-					}
-					return ".", off1, nil
-				}
 				break Loop
 			}
 			// literal string
@@ -479,6 +473,9 @@ Loop:
 	if ptr == 0 {
 		off1 = off
 	}
+	if len(s) == 0 {
+		s = []byte(".")
+	}
 	return string(s), off1, nil
 }