powerpc/pseries/hvcserver: don't memset pi_buff if it is null

pi_buff is being memset before it is sanity checked. Move the
memset after the null pi_buff sanity check to avoid an oops.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>