of1.2: fix OFPGroupDescStats parser

- add action's size to offset of bucket.

Signed-off-by: KONDOH Tasuku <kondoh.tasuku@po.ntts.co.jp>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>