internal/child_process: call postSend on error

Call `obj.postSend` in error case of `process.send()`. The
`net.Socket`'s handle should not be leaked.

Note that there are two callbacks invoked on handles
when they are sent to the child process over IPC pipes.
These callbacks are specified by `handleConversion` object, and
during send two of them are invoked:

  * `send`
  * `postSend`

Now for `net.Socket` in particular, `postSend` performs clean up by
closing the actual uv handle. However this clean up will not happen in
one of the branches. This pull request aims to fix this.

lib/internal/child_process.js

@@ -602,12 +602,18 @@ function setupChannel(target, channel) {
       } else {
         process.nextTick(function() { req.oncomplete(); });
       }
-    } else if (!swallowErrors) {
-      const ex = errnoException(err, 'write');
-      if (typeof callback === 'function') {
-        process.nextTick(callback, ex);
-      } else {
-        this.emit('error', ex);  // FIXME(bnoordhuis) Defer to next tick.
+    } else {
+      // Cleanup handle on error
+      if (obj && obj.postSend)
+        obj.postSend(handle);
+
+      if (!swallowErrors) {
+        const ex = errnoException(err, 'write');
+        if (typeof callback === 'function') {
+          process.nextTick(callback, ex);
+        } else {
+          this.emit('error', ex);  // FIXME(bnoordhuis) Defer to next tick.
+        }
       }
     }