x86: Do sanity test on pirq table before writing

If pirq_routing_table points to NULL, that means U-Boot fails to
generate the table before in create_pirq_routing_table(), so we
test it against NULL before actually writing it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>