child_process: don't emit same handle twice

It's possible to read multiple messages off the parent/child channel.
When that happens, make sure that recvHandle is cleared after emitting
the first message so it doesn't get emitted twice.

lib/child_process.js

@@ -337,6 +337,7 @@ function setupChannel(target, channel) {
         var message = JSON.parse(json);
 
         handleMessage(target, message, recvHandle);
+        recvHandle = undefined;
 
         start = i + 1;
       }

test/simple/test-child-process-fork-getconnections.js

@@ -35,6 +35,7 @@ if (process.argv[2] === 'child') {
     }
 
     if (m.cmd === 'close') {
+      assert.equal(socket, undefined);
       sockets[m.id].once('close', function() {
         process.send({ id: m.id, status: 'closed' });
       });