SQL: uninitialized read [#387]

rpl.rpl_events sometimes fails in release mode after be81b00c

This could be prevented with MSAN

sql/sql_class.cc

@@ -813,6 +813,8 @@ THD::THD(my_thread_id id, bool is_wsrep_applier, bool skip_global_sys_var_lock)
   // Must be reset to handle error with THD's created for init of mysqld
   lex->current_select= 0;
   start_utime= utime_after_query= 0;
+  system_time= 0;
+  system_time_sec_part= 0;
   utime_after_lock= 0L;
   progress.arena= 0;
   progress.report_to_client= 0;