MDEV-15456 Server crashes upon adding or dropping a partition in ALTER under LOCK TABLE after ER_SAME_NAME_PARTITION

ALTER TABLE ... ADD PARTITION modifies the open TABLE structure,
and sets table->need_reopen=1 to reset these modifications
in case of an error.

But under LOCK TABLES the table isn't get reopened, despite need_reopen.

Fixed by reopening need_reopen tables under LOCK TABLE.