virsh-nodedev: Avoid spurious errors

virsh-nodedev: Avoid spurious errors

Our public free functions explicitly don't accept NULL pointers
(sigh). Therefore, callers must do something like this:

    if (dev)
        virNodeDeviceFree(dev);

And we are not doing that on two places I've found. This leads to
dummy error message thrown by virsh:

    virsh # nodedev-dumpxml nonexistent-device
    error: Could not find matching device 'nonexistent-device'
    error: invalid node device pointer in virNodeDeviceFree

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>