fix bug in GitHubDriver

Since I updated my composer today using self-update, I can't update my vendors.

All my forked github private repos (configured as vcs repositories) give me this error : 

```
Permission denied (publickey).                                                                              
fatal: Could not read from remote repository. 
```

After investigating the issue, I think the problem was introduced in this PR https://github.com/composer/composer/pull/2375.
In the GitHubDriver the `$originUrl` is now set like this : 
```php
$this->originUrl = isset($match[1]) ? $match[1] : $match[2];
```
The problem is that `preg_match` sets `$match[1]`  to the empty string, not `null`. As a result, `$originUrl` is set to `""` instead of `"github.com"`.

By replacing the previous line by : 
```php
$this->originUrl = !empty($match[1]) ? $match[1] : $match[2];
```
the `originUrl` is set to `"github.com"` and everything works fine.

I am not at all familiar with Composer's code, so please excuse me if that's not a viable solution.