kernel/nata - Fix a memory leak in ata-dma.
kernel/natapci - Fix a NULL pointer dereference.

- ata_devclass is NULL when no unit 0/1 is available or no
  legacy support has been detected.
- device_add_child() populates ata_devclass via make_device()
  so do not try to get information out of it when NULL.

Submitted-by: @dillon