MDEV-12731

Was reported as 'rpl.rpl_gtid_stop_start fails with Valgrind in buildbot and outside'.
The 'Conditional jump or move depends on uninitialized value' valgrind complaint is valid
and means THD::m_current_stage_key that is not initialized indeed by constructor.

Fixed with its initialization added to the initializer list of
THD::THD.  Double checked with mysql to have found a similar fixes for
the very same issue which was discovered after P_S merge to Maria
(60589aee where the member is introduced) had been done.

sql/sql_class.cc

@@ -698,6 +698,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
              /* statement id */ 0),
    rli_fake(0), rgi_fake(0), rgi_slave(NULL),
    protocol_text(this), protocol_binary(this),
+   m_current_stage_key(0),
    in_sub_stmt(0), log_all_errors(0),
    binlog_unsafe_warning_flags(0),
    binlog_table_maps(0),