fix dumpConfig without credentials

this will only happen if you try and run the program from the command-line and your config either does not exist or doesn't have credentials...

```
$ npmd-config 

/usr/local/lib/node_modules/npmd-config/index.js:16
  _config.credentials.pass = _config.credentials.pass.replace(/./g,'*')
                                                ^
TypeError: Cannot read property 'pass' of undefined
    at dumpConfig (/usr/local/lib/node_modules/npmd-config/index.js:16:49)
    at Object.<anonymous> (/usr/local/lib/node_modules/npmd-config/index.js:108:3)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
```

cheers