[SCSI] gdth: Prevent negative offsets in ioctl CVE-2009-3080

A negative offset could be used to index before the event buffer and
lead to a security breach.
Signed-off-by: Dave Jones <davej@redhat.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/gdth.c

@@ -2900,7 +2900,7 @@ static int gdth_read_event(gdth_ha_str *ha, int handle, gdth_evt_str *estr)
         eindex = handle;
     estr->event_source = 0;
 
-    if (eindex >= MAX_EVENTS) {
+    if (eindex < 0 || eindex >= MAX_EVENTS) {
         spin_unlock_irqrestore(&ha->smp_lock, flags);
         return eindex;
     }