hmp: fix "dump-quest-memory" segfault

Running QEMU with "qemu-system-x86_64 -M none -nographic -m 256" and executing
"dump-guest-memory /dev/null 0 8192" results in segfault.
Fix by checking if we have CPU.

Signed-off-by: Iwona Kotlarska <iwona260909@gmail.com>
Message-Id: <20170330050924.22134-1-iwona260909@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
   Fixed up title