Undefined variable: host

In `db_connect()`, if the condition `while ( ( $group_key = array_shift( $this->unused_servers[ $dbhname ] ) ) ) {` is never met, `$host` and `$port` will be undefined when the `$error_details` array is created, generating a PHP notice.

It could be fixed by just adding:

```
if ( ! isset( $host ) ) { $host = ''; }
if ( ! isset( $port ) ) { $port = ''; }
```

before `$error_details`, but I don't know if that would be masking the root issue (should the `while` loop always execute at least once?) or not.

Start in db_connect() and reproduce the path where the while condition never succeeds. Inspect how $error_details uses $host and $port, then determine whether the loop should execute or whether unset values need handling; done means the notice is resolved without masking the underlying connection behavior.

Written by the indexing model from the issue text.