Allow the user who calls doadump() from the kernel debugger
to not get a page fault if he has not defined a dump device.
Panic can often not do a dump as it can hang forever in some cases.
The original PR was for amd64 only. This is a generalised version of
that change.

PR: amd[[ https://bugs.freebsd.org/64 | 64 ]]/67712
Submitted by: wjw@withagen.nl <Willen Jan Withagen>