scsi_disk_emulate_command passes in_buf == NULL when sent a REQUEST
SENSE command. Check for in_len == 0 before dereferencing in_buf.
Fixes: f68d98b21fa74155dc7c1fd212474379ac3c7531
Reported-by: Roman Kagan <rkagan@virtuozzo.com>
Tested-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>