ppc: fix double-free in cpu_post_load()

When running nested with KVM PR, ppc_set_compat() fails and QEMU crashes
because of "double free or corruption (!prev)". The crash happens because
error_report_err() has already called error_free().

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
