ASAN showed use-after-free in binlog_online_alter_end_trans, during
running through thd->online_alter_cache_list.

In online_alter_binlog_get_cache_data, new_cache_data was allocated on
thd->mem_root, in case of autocommit=1, but this mem_root could be freed
in sp_head::execute, upon using stored functions.

It appears that thd->transaction->mem_root exists even in single-stmt
transaction mode (i.e autocommit=1), so it can be used in all cases.
This mem_root will remain valid till the end of transaction, including
commit phase.