Bug #24976897: UNINITIALIZED MEMBER M_CURRENT_STAGE_KEY IN THD.

Seen after WL#9457 when running tests making use of the query cache.

Author: ssorumgard

sql/sql_class.cc

@@ -1061,6 +1061,7 @@ THD::THD(bool enable_plugins)
  initial_status_var(NULL),
  status_var_aggregated(false),
  query_plan(this),
+ m_current_stage_key(0),
  current_mutex(NULL),
  current_cond(NULL),
  in_sub_stmt(0),