core/cpu: fix next_ungarded_primary
core/cpu: fix next_ungarded_primary

next_unguarded_primary dereferences NULL CPU -> UB -> infinite loop

Fast reboot works again after this patch.

Fixes: 98f5834253c7e ("cpu: Keep track of the "ec_primary" in big core more")
Signed-off-by: Nicholas Piggin <24829078df5003d9dd1dd871ca2aeb0b1cf95ef4@gmail.com>
Signed-off-by: Vasant Hegde <a6d753843573f7bfed5075c1e875b0be8007a0ab@linux.vnet.ibm.com>

qemu/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot