child_process: add safety checks on stdio access

When a child process is spawned, there is no guarantee that stdout
and stderr will be created successfully. This commit adds checks
before attempting to access the streams.

PR-URL: https://github.com/nodejs/node/pull/3799
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/child_process.js

@@ -222,13 +222,21 @@ exports.execFile = function(file /*, args, options, callback*/) {
 
   function errorhandler(e) {
     ex = e;
+
+    if (child.stdout)
       child.stdout.destroy();
+
+    if (child.stderr)
       child.stderr.destroy();
+
     exithandler();
   }
 
   function kill() {
+    if (child.stdout)
       child.stdout.destroy();
+
+    if (child.stderr)
       child.stderr.destroy();
 
     killed = true;
@@ -247,6 +255,10 @@ exports.execFile = function(file /*, args, options, callback*/) {
     }, options.timeout);
   }
 
+  if (child.stdout) {
+    if (encoding)
+      child.stdout.setEncoding(encoding);
+
     child.stdout.addListener('data', function(chunk) {
       stdoutLen += chunk.length;
 
@@ -260,6 +272,11 @@ exports.execFile = function(file /*, args, options, callback*/) {
           _stdout += chunk;
       }
     });
+  }
+
+  if (child.stderr) {
+    if (encoding)
+      child.stderr.setEncoding(encoding);
 
     child.stderr.addListener('data', function(chunk) {
       stderrLen += chunk.length;
@@ -274,10 +291,6 @@ exports.execFile = function(file /*, args, options, callback*/) {
           _stderr += chunk;
       }
     });
-
-  if (encoding) {
-    child.stderr.setEncoding(encoding);
-    child.stdout.setEncoding(encoding);
   }
 
   child.addListener('close', exithandler);