Prcb may be set to 0 for some CPUs if the dump was taken before they
start. The dump may still contain valuable information for started CPUs
so don't abandon conversion in such a case.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
Message-id: 20230611033434.14659-1-akihiko.odaki@daynix.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>