Bug 212114 - loader: zio_checksum_verify() must test spa for NULL pointer
The issue was introduced with adding support for salted checksums, and
was revealed by bhyve userboot.so.
During pool discovery the loader is reading pool label from disks, and
at that time the spa structure is not yet set up, so the NULL pointer
is passed for spa. This condition must be checked to avoid the corruption
of the memory and NULL pointer dereference.