forms: Pass array of length equal to the size of the collection

Change-Id: Id880953105f6100b15cd78f48ce0c8dbcb7ffb8d
Reviewed-on: https://gerrit.libreoffice.org/11687
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>