sync: Check values before applying changes

In SyncInitTrigger(), we would set the CheckTrigger function before
validating the counter value.

As a result, if the counter value overflowed, we would leave the
function SyncInitTrigger() with the CheckTrigger applied but without
updating the trigger object.

To avoid that issue, move the portion of code checking for the trigger
check value before updating the CheckTrigger function.

Related to CVE-2025-26601, ZDI-CAN-25870

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Part-of: <!1828>