panic on shutdown

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

Two days running, a nearly -current (Dec 7) 6.99.28/amd64 box
panics on shutdown with

System panicked: kernel diagnostic assertion "sdp != NULL" failed: file 
"/usr/src/sys/uvm/uvm_swap.c", line 1683 

     $NetBSD: uvm_swap.c,v 1.165 2013/11/23 14:50:40 christos Exp $

_KERNEL_OPT_NARCNET() at 0
?() at fffffe807e886cd0
vpanic() at vpanic+0x13f
_etext() at _etext
uvm_swap_markbad() at uvm_swap_markbad+0x78
uvmfault_anonget() at uvmfault_anonget+0x34d
uvm_fault_internal() at uvm_fault_internal+0x103e
trap() at trap+0x4fa
--- trap (number 6) ---
7f7ff6d0b95c:

On both occasions said box had been doing a lot of pkgsrc building
with a tmpfs WRKOBJDIR which would have been quite full, and backed
by swap. The box runs fine otherwise, the panic just happens on
shutdown...

Cheers,

Patrick