(ok 3.4.4) "Create an index on X columns" form not validated

To reproduce:

* go to a db's structure page
* scroll to the the part of the page that deal with indices
* in the "Create an index on X columns" form type a value of 0
* click "Go"

expected result:
* a javaScript warning saying that the value must be at least 1 and the form not submitted

actual result:
* no warning and the form is submitted, this is followed by a new page with no rows in the indices table