Cleanups:

- unused variable THD::con_root removed
- we don't need to bzero(&warn_root) just before init_alloc_root(&warn_root)
- we don't need to bzero(&transaction.mem_root) because we 
  bzero(&transaction) later in THD::THD
- we don't need to free thd->mem_root thd->transaction.mem_root
  in handle_one_connection because they are freed in THD::~THD
  which is called in end_thread later 

sql/sql_class.cc

@@ -151,9 +151,6 @@ THD::THD():user_time(0), is_fatal_error(0),
   init();
   /* Initialize sub structures */
   bzero((char*) &mem_root,sizeof(mem_root));
-  bzero((char*) &transaction.mem_root,sizeof(transaction.mem_root));
-  bzero((char*) &con_root,sizeof(con_root));
-  bzero((char*) &warn_root,sizeof(warn_root));
   init_alloc_root(&warn_root, WARN_ALLOC_BLOCK_SIZE, WARN_ALLOC_PREALLOC_SIZE);
   user_connect=(USER_CONN *)0;
   hash_init(&user_vars, system_charset_info, USER_VARS_HASH_SIZE, 0, 0,
@@ -355,7 +352,6 @@ THD::~THD()
   safeFree(db);
   safeFree(ip);
   free_root(&mem_root,MYF(0));
-  free_root(&con_root,MYF(0));
   free_root(&warn_root,MYF(0));
   free_root(&transaction.mem_root,MYF(0));
   mysys_var=0;					// Safety (shouldn't be needed)

sql/sql_class.h

@@ -452,7 +452,6 @@ public:
   LEX     main_lex;
   LEX	  *lex;				// parse tree descriptor
   MEM_ROOT mem_root;			// 1 command-life memory pool
-  MEM_ROOT con_root;                    // connection-life memory
   MEM_ROOT warn_root;			// For warnings and errors
   Protocol *protocol;			// Current protocol
   Protocol_simple protocol_simple;	// Normal protocol

sql/sql_parse.cc

@@ -957,7 +957,6 @@ pthread_handler_decl(handle_one_connection,arg)
     }
     if (thd->user_connect)
       decrease_user_connections(thd->user_connect);
-    free_root(&thd->mem_root,MYF(0));
     if (net->error && net->vio != 0 && net->report_error)
     {
       if (!thd->killed && thd->variables.log_warnings)
@@ -1537,8 +1536,6 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
 #endif
     close_connection(thd, 0, 1);
     close_thread_tables(thd);			// Free before kill
-    free_root(&thd->mem_root,MYF(0));
-    free_root(&thd->transaction.mem_root,MYF(0));
     kill_mysql();
     error=TRUE;
     break;