arm64: acpi: Harden get_cpu_for_acpi_id() against missing CPU entry

In the Linux kernel, the following vulnerability has been resolved: arm64: acpi: Harden get_cpu_for_acpi_id() against missing CPU entry In a review discussion of the changes to support vCPU hotplug where a check was added on the GICC being enabled if was online, it was noted that there is need to map back to the cpu and use that to index into a cpumask. As such, a valid ID is needed. If an MPIDR check fails in acpi_map_gic_cpu_interface() it is possible for the entry in cpu_madt_gicc[cpu] == NULL. This function would then cause a NULL pointer dereference. Whilst a path to trigger this has not been established, harden this caller against the possibility.