install/56893: Adding a NetBSD partition to existing GPT partitions fails in sysinst

[code-netbsd%boerschig.net@localhost]

>Number:         56893
>Category:       install
>Synopsis:       Adding a NetBSD partition to existing GPT partitions fails in sysinst
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jun 19 17:50:00 +0000 2022
>Originator:     MB
>Release:        9.99.97
>Organization:
>Environment:
NetBSD 9.99.97 (GENERIC) amd64 Sat Jun 18 13:56:41 UTC 2022 

>Description:
Not sure if this is a supported installation type.
Trying to install here on a NVME with an existing GPT (1 EFI system partition, 1 Linux partition) on a laptop.
It fails in sysinst in the partitioning screen: 

the partition size is always invalid (looks like (int64_t)-1 with some values added, see below), sysinst reports "Too large!" for any value entered by the user, and "Invalid numeric" on entering -1.
The dialog pane reads something like:

Invalid Numeric!

Special values that can be entered for the size value:
   -1: use until the end
  a-b: use until the given partition

Size (max 18446744073709542912 MB) [18446744073709542912]: 
>How-To-Repeat:
First encountered on a laptop, can be reproduced in QEMU with similar partition setup:

#host
qemu-img create netbsd-current.img 20G
qemu-system-x86_64 -cdrom NetBSD-9.99.97-amd64.iso  -drive if=none,format=raw,file=netbsd-current.img,id=hd0,cache=writeback -device virtio-blk-pci,drive=hd0   -boot d

# In Qemu:
# exit sysinst
^C
# add the GPT partitions similar to the laptop's
gpt create /dev/ld0
gpt add -b 2k -i 1 -t efi -l efi_system -s 512M ld0
gpt add -i 2 -t linux-data -l some_linux -s 8G ld0

# go back to sysinst, try to add a partition after the linux-data partition
# e.g. new partition start: 8705 MB, size: until the end

^D 
>Fix: