Avoid a recursive kernel fault and subsequent double fault if the VM fault

Avoid a recursive kernel fault and subsequent double fault if the VM fault
code gets a KVM map_entry with a NULL object.  Such entries exist in system
maps managed directly by the kernel, such as the buffer cache and kernel_map.
Instead, we check for the condition and panic immediately.  Programs which
access /dev/[k]mem can hit this race/failure.

Reported-by: =?ISO-8859-1?Q?Stefan_Kr=FCger?= <skrueger@meinberlikomm.de>