install/60333: sysinst(8): reproducible core dump when use default partitions selected on installation disk

["vezhlys%gmail.com@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60333
>Category:       install
>Synopsis:       sysinst(8): reproducible core dump when use default partitions selected on installation disk
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 15 07:55:00 +0000 2026
>Originator:     Andrius V
>Release:        NetBSD 11.0 RC4
>Organization:
>Environment:
>Description:
There's an easily reproducible core dump when install disk is chosen as the one to install to and "Use default partition sizes" is selected.

I assume it is a bit invalid case use case, since the choice is likely trying to destroy root partition, nevertheless available to the user, and maybe it exposes some other bugs too? 

After the selection sysinst crash with error messages below:
assertion "parts->num_part == 0" failed: file "/usr/src/usr.sbin/sysinst/arch/amd64/../../bsddisklabel.c", line 1738, function "replace_by_default"
[1] Abort trap (core dump) ${cmd}


>How-To-Repeat:
Boot on amd64 USB flash drive flash drive (tried on BIOS boot)

Select options:
Installation messages in English
unchanged
Install NetBSD to hard disk
Yes
sd0 (install disk in my case)
This is the correct geometry
Use default partition sizes
Observe sysinst crash
>Fix: