Fix a problem that could cause a spurious SQLITE_NOMEM error when attempting

Fix a problem that could cause a spurious SQLITE_NOMEM error when attempting
to resume an RBU operation if the previous client failed right after
completing the incremental checkpoint. Also a "cannot vacuum wal db" error
that could occur when resuming an RBU vacuum if an error (OOM or IO error)
occurs during the incremental checkpoint.

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