fix compilation with gcc-10

../sql/sql_class.cc: In constructor ‘start_new_trans::start_new_trans(THD*)’:
../include/m_string.h:61:49: error: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘struct Ha_data’; use assignment or value-initialization instead [-Werror=class-memaccess]
   61 | # define bzero(A,B)             memset((A),0,(B))
      |                                                 ^
../sql/sql_class.cc:5805:3: note: in expansion of macro ‘bzero’
 5805 |   bzero(thd->ha_data, sizeof(thd->ha_data));
      |   ^~~~~
In file included from ../sql/sql_class.cc:33:
../sql/sql_class.h:2003:8: note: ‘struct Ha_data’ declared here
 2003 | struct Ha_data
      |        ^~~~~~~

sql/sql_class.cc

@@ -5802,7 +5802,8 @@ start_new_trans::start_new_trans(THD *thd)
   mdl_savepoint= thd->mdl_context.mdl_savepoint();
   memcpy(old_ha_data, thd->ha_data, sizeof(old_ha_data));
   thd->reset_n_backup_open_tables_state(&open_tables_state_backup);
-  bzero(thd->ha_data, sizeof(thd->ha_data));
+  for (auto &data : thd->ha_data)
+    data.reset();
   old_transaction= thd->transaction;
   thd->transaction= &new_transaction;
   new_transaction.on= 1;

sql/sql_class.h

@@ -2024,6 +2024,14 @@ struct Ha_data
   */
   plugin_ref lock;
   Ha_data() :ha_ptr(NULL) {}
+
+  void reset()
+  {
+    ha_ptr= nullptr;
+    for (auto &info : ha_info)
+      info.reset();
+    lock= nullptr;
+  }
 };
 
 /**