SelectAttribute should also reset postValue in setChoices

In SelectAttribute::setChoices, we check that $this->value is in the list of authorized choices, but we do not check $this->postValue. This causes problems in the edgecase where setChoices is called in-between readPostValue and applyPostValue, leaving the field with a invalid value despite the call to setChoices.

It is a problem for CSV import template selection field.