Add check to abort if $argv is undefined.

My shell worked great locally when invoked with `path/app/Console/cake shell arg`. When I tried to start it from a cron job on the server, it took the server down by creating an enormous error_log file.

```
PHP Notice:  Undefined variable: argv in path/app/Console/cake.php on line 35
PHP Warning:  array_search() expects parameter 2 to be array, null given in path/lib/Cake/Console/ShellDispatcher.php on line 327
... (ad inifinitum)
```

This doesn't fix the issue but at least prevents the runaway process and exits with an informative error message. The solution for me was to invoke cake.php directly with `php-cli`.