9772 Panic in ahci when the failed slot spkt is NULL

ahci_dump_commands() is called from ahci_intr_fatal_error().

When deciding whether to log the error, ahci_intr_fatal_error() looks up the spkt and checks that it is not NULL and if it meets certain criteria.

However, if it is NULL then there is still a code path where ahci_dump_commands() is called with instructions to dump the command stored in the slot.
Fix (i.e. no more kernel panic) confirmed by affected user.