install/55045: Assertion failure in _strlcpy booting i386 install media

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55045
>Category:       install
>Synopsis:       Assertion failure in _strlcpy booting i386 install media
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 04 10:25:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.02.19.21.51.21
>Organization:

>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

Booting the NetBSD-current/i386 install media, an assertion failure
is logged before the first sysinst output appears:

  Created tmpfs /dev (1720320 byte, 3328 inodes)
  erase ^?, werase ^W, kill ^U, intr ^C
  assertion "dst != NULL" failed: file "/tmp/bracket/build/2020.02.22.02.28.06-i386/src/lib/libc/../../common/lib/libc/string/strlcpy.c", line 60, function "_strlcpy"

Despite the message, sysinst starts and the installation succeeds.

The problem started with these commits:

  2020.02.19.21.42.53 martin src/usr.sbin/sysinst/Makefile.inc 1.32
  2020.02.19.21.42.54 martin src/share/man/man8/Attic/sysinst.8 1.7
  2020.02.19.21.42.54 martin src/share/man/man8/Makefile 1.110
  2020.02.19.21.42.54 martin src/usr.sbin/sysinst/sysinst.8 1.1
  2020.02.19.21.45.09 martin src/usr.sbin/sysinst/Makefile.inc 1.33
  2020.02.19.21.45.09 martin src/usr.sbin/sysinst/disklabel.c 1.37
  2020.02.19.21.49.48 martin src/usr.sbin/sysinst/Makefile.inc 1.34
  2020.02.19.21.51.21 martin src/usr.sbin/sysinst/defs.h 1.56
  2020.02.19.21.51.21 martin src/usr.sbin/sysinst/main.c 1.22
  2020.02.19.21.51.21 martin src/usr.sbin/sysinst/sysinst.8 1.2

The problem affects the TNF i386 testbed (floppy install in qemu) and
my own bare metal i386 testbed (network install on real hardware).
The amd64 port does not appear to be affected.

>How-To-Repeat:

Install NetBSD/i386 using a serial console and log the serial console
output.

>Fix: