terminate MX label with null

packer/type.mx.js

@@ -22,7 +22,7 @@ exports.DNS_PACKER_TYPE_MX = function (ab, dv, total, record) {
   // 16-bit priority and a sequence of labels as the exchange
   record.exchange.split('.').forEach(function (label) {
     mxLen += 1 + label.length;
-    
+
     dv.setUint8(total, label.length, false);
     total += 1;
 
@@ -31,10 +31,13 @@ exports.DNS_PACKER_TYPE_MX = function (ab, dv, total, record) {
       total += 1;
     });
   });
+  dv.setUint8(total, 0, false);
+  total += 1;
+  mxLen += 1;
 
   // RDLENGTH
   dv.setUint16(rdLenIndex, mxLen, false);
-  
+
   return total;
 };