dgram: use Buffer.alloc(0) for zero-size buffers

There is no difference between alloc(0) and allocUnsafe(0), so there is
no reason to confuse anyone reading the code with an additional call to
allocUnsafe.

PR-URL: https://github.com/nodejs/node/pull/8751
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/dgram.js

@@ -350,7 +350,7 @@ Socket.prototype.send = function(buffer,
     self.bind({port: 0, exclusive: true}, null);
 
   if (list.length === 0)
-    list.push(Buffer.allocUnsafe(0));
+    list.push(Buffer.alloc(0));
 
   // If the socket hasn't been bound yet, push the outbound packet onto the
   // send queue and send after binding is complete.