skiboot/hw/imc: Add NULL pointer check
skiboot/hw/imc: Add NULL pointer check

Minor cleanup to avoid null pointer access.

Signed-off-by: Madhavan Srinivasan <50c9fed97c2a760bbf32e7fdccf39ca6a997eabd@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

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