[SCSI] sg: fix /proc/scsi/sg/devices when no SCSI devices

The patch "[SCSI] sg: use idr to replace static arrays" in 2.6.24-rc1
causes a bogus line to appear in /proc/scsi/sg/devices containing
"-1 -1 -1 -1 -1 -1 -1 -1 -1" when there are no SCSI devices in the
system.  In 2.6.23, /proc/scsi/sg/devices is empty when there are no
SCSI devices in the system.  A similar problem exists with
/proc/scsi/sg/device_strs.  The following patch restores the behavior
of 2.6.23.

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>