child_process: fix sending handle twice

When sending a socket to a child process via IPC pipe,
`child_process.js` picks a raw UV handle from `_handle` property, sends
it, and assigns `null` to the property. Sending the same socket twice
was resulting in a runtime error, since we weren't handling the empty
`_handle` case.

In case of `null` `_handle` we should send just a plain text message
as passed it was passed to `.send()` and ignore the handle, letting
users handle such cases themselves instead of throwing the error at
runtime.

fix #5469

lib/child_process.js

@@ -111,6 +111,9 @@ var handleConversion = {
 
   'net.Socket': {
     send: function(message, socket) {
+      if (!socket._handle)
+        return;
+
       // if the socket was created by net.Server
       if (socket.server) {
         // the slave should keep track of the socket
@@ -139,7 +142,8 @@ var handleConversion = {
 
     postSend: function(handle) {
       // Close the Socket handle after sending it
-      handle.close();
+      if (handle)
+        handle.close();
     },
 
     got: function(message, handle, emit) {
@@ -438,6 +442,11 @@ function setupChannel(target, channel) {
       // convert TCP object to native handle object
       handle = handleConversion[message.type].send.apply(target, arguments);
 
+      // If handle was sent twice, or it is impossible to get native handle
+      // out of it - just send a text without the handle.
+      if (!handle)
+        message = message.msg;
+
       // Update simultaneous accepts on Windows
       if (obj.simultaneousAccepts) {
         net._setSimultaneousAccepts(handle);