make multiselectfield init more robust

[Cesar48]

In a newer version of chafield, the max length is no longer required and the validator may be missing. This code should fix that. Minor note I want to add: the init code of Field actually caused the maxlen validator to be last, not first, so you'd need to always take [-1] instead of [0] or you'd overwrite user-supplied validators (but that's also fixed here)

[Cesar48]

Also see #132, fixes the same problem in a different way

[blag]

Fixed in #148.