blk-throttle: fix potential NULL pointer dereference in throtl_select_dispatch

tg in throtl_select_dispatch is used first and then do check. Since tg
may be NULL, it has potential NULL pointer dereference risk. So fix
it.
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -1207,7 +1207,7 @@ static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
 
 	while (1) {
 		struct throtl_grp *tg = throtl_rb_first(parent_sq);
-		struct throtl_service_queue *sq = &tg->service_queue;
+		struct throtl_service_queue *sq;
 
 		if (!tg)
 			break;
@@ -1219,6 +1219,7 @@ static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
 
 		nr_disp += throtl_dispatch_tg(tg);
 
+		sq = &tg->service_queue;
 		if (sq->nr_queued[0] || sq->nr_queued[1])
 			tg_update_disptime(tg);