[POWERPC] Extra sanity check in EEH code

Don't dereference a device node that isn't there.  A "shouldn't
happen" case, but someone ran into it with a possibly misconfigured
device tree.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>