ibuf_init_at_db_start(): Validate the change buffer root page.
A later version may stop creating a change buffer, and this
validation check will prevent a downgrade from such later versions.

ibuf_max_size_update(): If the change buffer was not loaded, do nothing.

dict_boot(): Merge the local variable "error" to "err". Ignore
failures of ibuf_init_at_db_start() if innodb_force_recovery>=4.