child_process: reuse existing no-op function

The internal/child_process module has an existing no-op
function. This commit utilizes that function, instead of
creating extraneous closures.

PR-URL: https://github.com/nodejs/node/pull/8164
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

lib/internal/child_process.js

@@ -87,7 +87,7 @@ const handleConversion = {
       // remove handle from socket object, it will be closed when the socket
       // will be sent
       if (!options.keepOpen) {
-        handle.onread = function() {};
+        handle.onread = nop;
         socket._handle = null;
       }