arena: Fix initialization of "warned_" member
arena: Fix initialization of "warned_" member

The warned_ member was uninitialized by one of the constructors of
ArenaBase, causing the "big arena" warning to be missed sometimes.
This is difficult to test for, but was caught by UBSAN.

Change-Id: If6eee846669f3cb9ce988bb5d1322e8d8a214352
Reviewed-on: http://gerrit.ent.cloudera.com:8080/831
Reviewed-by: Matteo Bertozzi <1d5ca967930bde0bdce795acbf71a5e281d2d4e5@cloudera.com>
Tested-by: jenkins

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