exec() breaks if executed in a deleted directory

Sample:

``` js
$ node
> sh = require('shelljs')
> sh.mkdir(sh.tempdir() + '/deleteme')
> sh.cd(sh.tempdir() + '/deleteme')
> sh.rm('-rf', sh.tempdir() + '/deleteme')
> sh.exec('echo hi')
```

This works in bash:

``` bash
$ mkdir /tmp/deleteme
$ cd /tmp/deleteme/
$ rm -rf /tmp/deleteme/
$ echo hi
hi
```
