core: panic_stop: print current cell only if it has been set

core: panic_stop: print current cell only if it has been set

Currently during a panic, panic_stop will print the current cell
on the CPU where the panic occurred. However, if the hypervisor
panics sufficiently early during initialization, we may end up in
a situation where the root cell has not been initialized. This can
easily cause a trap loop, making the panic output less useful.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>