openpic: don't crash on a register access without a CPU context

If we access a register via the QEMU memory inspection commands (e.g.
"xp") rather than from guest code, we won't have a CPU context.
Gracefully fail to access the register in that case, rather than
crashing.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
