esas2r: Fix array overrun

Check the array size *before* dereferencing it with a user provided
offset.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>