child_process: Fix stdout=null when stdio=['pipe']

Previously, a command with a short stdio array would result in the child's
stdout and stderr objects set to null. For example:

var c = child_process.spawn(cmd, args, {stdio: ['pipe']});
// results in c.stdout === null.

The expected behavior is the above line functioning the same as this one:

var c = child_process.spawn(cmd, args, {stdio: ['pipe', null, null]});
// provides correct (non-null) c.stdout; as does the above, after this fix.