fix "Cannot read property 'scriptPubKey' of undefined" error

For some zcash-based coins or kmd-assets is needed to handle
a tx without any inputs or outputs, here we do additional check
if vout[0] is exists.

lib/explorer.js

@@ -307,7 +307,12 @@ module.exports = {
         loop.next();
       }
     }, function(){
-      if (vout[0].scriptPubKey.type == 'nonstandard') {
+
+       if (!vout[0]) {
+		return cb(arr_vout, arr_vin);
+       }
+
+       if (vout[0].scriptPubKey.type == 'nonstandard') {
         if ( arr_vin.length > 0 && arr_vout.length > 0 ) {
           if (arr_vin[0].addresses == arr_vout[0].addresses) {
             //PoS