Fix arena.cc compilation in DEBUG mode
Fix arena.cc compilation in DEBUG mode

Earlier commit 59b77edb887c1f389a627a6c20488094a584891b had only been
tested in release mode, forgot to update some #ifndef NDEBUG checks.

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