Users without set expiration permissions unable to save expirable nodes

node_expire's form_alter validation always checks the value of node->expire, even when the authenticated user doesn't have permission to edit expiration dates.

Therefore when a user tries to save an expireable node type, they are greeted with the unfriendly "You have to specify a valid date." in reference to a #value field which doesn't even appear on the form.

Marking this critical since it effectively makes this module unusable for non-admins.