powerpc/pmac: Fix SMP kernels on pre-core99 UP machines

commit 78c5c68a4cf4329d17abfa469345ddf323d4fd62 upstream.

The code for "powersurge" SMP would kick in and cause a crash
at boot due to the lack of a NULL test.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Reported-by: Adam Conrad <adconrad@ubuntu.com>
Tested-by: Adam Conrad <adconrad@ubuntu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>