scsi: target: pscsi: Fix possible null-pointer dereference in pscsi_complete_cmd()

scsi: target: pscsi: Fix possible null-pointer dereference in pscsi_complete_cmd()

The return value of transport_kmap_data_sg() is assigned to the variable
buf:

  buf = transport_kmap_data_sg(cmd);

And then it is checked:

  if (!buf) {

This indicates that buf can be NULL. However, it is dereferenced in the
following statements:

  if (!(buf[3] & 0x80))
    buf[3] |= 0x80;
  if (!(buf[2] & 0x80))
    buf[2] |= 0x80;

To fix these possible null-pointer dereferences, dereference buf and call
transport_kunmap_data_sg() only when buf is not NULL.

Link: https://lore.kernel.org/r/20210810040414.248167-1-islituo@gmail.com
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Tuo Li <islituo@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>