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.

Author: montywi

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,

sql/sql_class.h

@@ -2630,10 +2630,14 @@ class THD: public THD_count, /* this must be first */
  free_root(&mem_root,MYF(MY_KEEP_PREALLOC));
  DBUG_VOID_RETURN;
  }
- my_bool is_active()
+ bool is_active()
  {
  return (all.ha_list != NULL);
  }
+ bool is_empty()
+ {
+ return all.is_empty() && stmt.is_empty();
+ }
  st_transactions()
  {
  bzero((char*)this, sizeof(*this));