kern/53311: boot netbsd -d panics

[prlw1%cam.ac.uk@localhost]

>Number:         53311
>Category:       kern
>Synopsis:       boot netbsd -d panics
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu May 24 13:00:00 +0000 2018
>Originator:     Patrick Welche
>Release:        NetBSD-8.99.18/amd64
>Organization:
>Environment:
>Description:
boot netbsd -vx        works
boot netbsd -d         panics, but no serial console on either machine
                       so don't know what

One machine had ordinary old bootblocks, the other fresh netboot pxeboot_ia32.bin. ctrl-alt-esc does drop into ddb.
>How-To-Repeat:
boot netbsd -d
>Fix: