buf: Avoid calling bufdomain() on newly initialized bufs

[markj]

Summary

bufinit() inserts newly initialized bufs into the QUEUE_EMPTY queue, at
which point they haven't yet been assigned a domain. Thus, bufdomain()
returns &bdomain[-1], which trips the array-bounds sanitizer.

This is harmless since we don't use the result in that case, but let's
avoid the invalid access to begin with. This is sufficient to let an
amd64 kernel boot to a login prompt with -fsanitize=array-bounds
configured.

Reported by: Andrew Griffiths <[e-mail]>

[rlibby]

Looks good.