dhcp server null pointer in option_cache_dereference

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

On some machines newly upgraded to 9.0_STABLE, dhcp runs fine for, say, 24 
hours then starts logging the following. After this point, it no longer 
handles any DHCP requests:

Null pointer in option_cache_dereference: 
/usr/src/external/mpl/dhcp/bin/server/../../dist/server/dhcp.c(4067)

That 4067 is meant to be a line number according to dist/common/options.c, 
but there's no call to option_cache_dereference() at line 4067. Also, most 
calls to option_cache_dereference() don't specify the line as the third 
option, but I may be missing something.

Any ideas?

--
Stephen