linked_list-test-util: quelch warning
linked_list-test-util: quelch warning

I saw this when building in release mode:

../../src/kudu/util/status.h: In member function ‘void kudu::ScopedRowUpdater::RowUpdaterThread()’:
../../src/kudu/util/status.h:91:40: warning: ‘next_key’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     const ::kudu::Status& _s = (to_call);                   \
                                        ^
In file included from ../../src/kudu/integration-tests/linked_list-test.cc:50:0:
../../src/kudu/integration-tests/linked_list-test-util.h:288:13: note: ‘next_key’ was declared here
     int64_t next_key;
             ^

Change-Id: Ic2f6d9a96b76f1e7c587a53037856001bbda9ea7
Reviewed-on: http://gerrit.cloudera.org:8080/15479
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu