fix a small bug that could stop the check link process.

Just a small fix for the issue below during link checks:

```
Error: undefined method `+' for nil:NilClass
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/usr/local/Cellar/brew-cask/0.18.2/rubylib/cask/link_checker.rb:55:in `_get_data_from_request'
/usr/local/Cellar/brew-cask/0.18.2/rubylib/cask/link_checker.rb:19:in `run'
/usr/local/Cellar/brew-cask/0.18.2/rubylib/cask/cli/checklinks.rb:6:in `run'
/usr/local/Cellar/brew-cask/0.18.2/rubylib/cask/cli/checklinks.rb:4:in `each'
/usr/local/Cellar/brew-cask/0.18.2/rubylib/cask/cli/checklinks.rb:4:in `run'
/usr/local/Cellar/brew-cask/0.18.2/rubylib/cask/cli.rb:35:in `process'
/usr/local/bin/brew-cask.rb:6
/usr/local/Library/brew.rb:51:in `require'
/usr/local/Library/brew.rb:51:in `require?'
/usr/local/Library/brew.rb:101
```