arena: fix another bug referring to unused memory
arena: fix another bug referring to unused memory

In DEBUG mode, was referring back to the previously last component
during Reset() after it had already been de-allocated

(caught by unit tests when compiled by clang++)

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