[SCSI] aacraid: check buffer address in aac_internal_transfer

Captured a panic on an older kernel where an application issuing
commands via sg was sending requests that lacked a request_buffer, thus
the buffer pointer used in aac_internal_transer was NULL. The
application was fixed closing the issue, but felt it was advised to
immunize the driver against the eventuality.

Signed-off-by: Mark Salyzyn <aacraid@adaptec.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>