Added asssert to init_of_queries() to make it more safe

Part of review of 10.4 code by Sergei.

Other things:

- Changed return type if is_active() from my_bool to bool as this is
  only used by C++ code.

sql/sql_class.cc

@@ -1386,6 +1386,7 @@ void THD::init_for_queries()
     We don't need to call ha_enable_transaction() as we can't have
     any active transactions that has to be committed
   */
+  DBUG_ASSERT(transaction.is_empty());
   transaction.on= TRUE;
 
   reset_root_defaults(mem_root, variables.query_alloc_block_size,