panic: rcv_start < rcv_end

[Yuri Pankov]

Just seen this almost immediately after booting the system installed 
from amd64-20190906-r351901 snapshot, trying to do initial pkg 
bootstrap.  Sadly, I didn't have the swap/dump device configured at the 
time, so no dump was saved.

But it looks like I'm not alone, seeing the 
https://forums.freebsd.org/threads/kernel-panic-on-bhyve-virtualization.72222/ 
topic.  Note that I'm running on bare metal, so bhyve isn't involved. 
My panic screenshot is at https://pasteboard.co/IwLaXXb.jpg.

In (the most likely) case it's not helpful enough, I'm now running with 
dump device configured, and will update if/when the panic reproduces.