ipa-otpd: initialize local pointers and handle gcc 10
    
    oauth2_on_child_readable() does not use the main verto context and used
    to drop the argument name to signify that. This is a feature of C2X
    standard by default and is not enabled in gcc before 11 by default (it
    is enabled in RHEL 8's gcc 8.5).
    
    Add a simple 'if the context is missing, get out' code to use 'ctx'.
    This allows to avoid enabling C2X features.
    
    Initialize local pointers to prevent use before initialization on exit
    paths in abnormal situations as well.
    
    Fixes: https://pagure.io/freeipa/issue/9230
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Stanislav Levin <slev@altlinux.org>