There were two issues:

* set_var_default_role::user was overwritten with a new value,
  allocated in the thd->mem_root, which is reset between executions.
  That was causing the crash. Solved by introducing set_var_default_role::real_user

* when privilege tables were opened on EXECUTE, the reprepare_observer
  would abort the statement (as privilege tables are opened using
  the local TABLE_LIST that doesn't preserve metadata from PREPARE, so
  reprepare_observer thought they're changed). This issue also applied
  to SET PASSWORD. Solved by disabling reprepare_observer.