fix RowBlock::row_slice() to return correct results
fix RowBlock::row_slice() to return correct results

Was previously always returning the first row.
Fixes test failure introduced by 9d54acc3ce250c37d5cbea4386b91fcb37277941

cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu