FIX 17.0: follow-up of PR #27008

[atm-florianm]

FIX $sInvoice->ref inconsistent with database value in some cases

This is the same problem as PR #27008: I had forgotten to also use $this->newref rather than $num to update $this->ref in that fix (because $num cannot be modified by trigger).

Here is a fictional example of the problem:

$sInvoice->fetch($draftinvoiceid);
// $sInvoice->ref === '(PROV01234)'
$sInvoice->validate();
// Dolibar core first sets ref to 'SI_TEST_01'
// Let's say a module implementing BILL_SUPPLIER_VALIDATE then changes the ref to 'SI_TEST_02' and calls $object->update()

var_dump($sInvoice->ref); // expected: 'SI_TEST_02', got: 'SI_TEST_01'