Fix bug: dropped table between save and restore causes ERROR on restore.

Steps to reproduce:

1) Enable pg_hibernator.
2) Create a table.
3) Stop database.
4) Disable pg_hibernator.
5) Start database.
6) Drop the table.
7) Stop the database.
8) Enable pg_hibernator.
9) Start the database.

At this point the Block Reader errors out because of this bug.
Final edits for submission to Postgres community.