Don't assume slt-fsp-pass-strength-result is set.

This fixes an issue we're having when users try to use your plugin with iTheme's Exchange. You shouldn't assume that your POST key is set as other plugins may access the WP function from elsewhere.

In our example, its being called via AJAX and the PHP warning is breaking the json for the callback.

Thanks!
Merge pull request #8 from glennansley/fix-empty-index

Thanks. As you'll see from the rest of the code, I usually check vars before using them, this was a slip. On the other hand, the function isn't intended to be used by other code. These days I use classes!