scsi: fix scsi_convert_sense crash when in_buf == NULL && in_len == 0

scsi_disk_emulate_command passes in_buf == NULL when sent a REQUEST
SENSE command.  Check for in_len == 0 before dereferencing in_buf.

Fixes: f68d98b2
Reported-by: Roman Kagan <rkagan@virtuozzo.com>
Tested-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>