DROP SERVER / ALTER SERVER under FLUSH TABLES WITH READ LOCK → assertion Diagnostics_area::set_ok_status (double-set of the diagnostics area)

On a debug build, in one session:
CREATE SERVER v0 FOREIGN DATA WRAPPER mysql OPTIONS (HOST 'h');
FLUSH TABLES WITH READ LOCK;
DROP SERVER v0;
Result:
mariadbd: /home/mysql/mariadb/sql/sql_error.cc:357:
void Diagnostics_area::set_ok_status(ulonglong, ulonglong,
const char *):
Assertion `!is_set() || (m_status == DA_OK_BULK &&
is_bulk_op())' failed.
… got signal 6 ;

1. Diagnostics_area::set_ok_status(...) sql/sql_error.cc:357
2. mysql_execute_command(THD*, bool)
   ALTER SERVER v0 OPTIONS (HOST 'h') instead of DROP SERVER
   triggers the same assertion.

Preconditions (each matters):

• The server must exist — DROP SERVER IF EXISTS <nonexistent>
  under the read lock does not crash (it bails before the write).
• The global read lock is the trigger — without FLUSH TABLES
  WITH READ LOCK, DROP SERVER v0 succeeds.