mariadb: use-after-poison in prepare_inplace_add_virtual in handler0alter.cc

MariaDB v10.4 to v10.7 was discovered to contain an use-after-poison in prepareinplaceaddvirtual at /storage/innobase/handler/handler0alter.cc...

MariaDB v10.4 to v10.7 was discovered to contain an use-after-poison in prepare_inplace_add_virtual at /storage/innobase/handler/handler0alter.cc.

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