Warnings and Notices on install

Hey soma - I just installed your fork on a new site and received these during install. All gone once the sites field was populated and submitted.

```
Warning: strpos(): Empty needle in /Users/ajones/Sites/pwnew/site/modules/Multisite/Multisite.module on line 138

Notice: Undefined index: subdomains in /Users/ajones/Sites/pwnew/site/modules/Multisite/Multisite.module on line 219
```
