lua: fix memory leak in error_object:set_prev()

By design, the error references its cause, see error_set_prev() in diag.c.
Referencing the error from error_set_prev() in error.lua is wrong.
Introduced by commit 2e3c81de ("error: use int64_t as reference
counter").

Closes #9694

NO_DOC=bugfix
NO_TEST=memory leak

(cherry picked from commit d08dbcbf)