pg_dump throwing "column number -1 is out of range 0..36" on HEAD

Hi all,

Trying to do pg_dump[all] on a 9.5 or older server results in spurious
failures:
pg_dump: column number -1 is out of range 0..36

After looking around, the problem comes from
check_tuple_field_number(), more specifically from getTables() where
someone has forgotten to add NULL values for amname when querying
older server versions.

Attached is a patch to fix that.  I am not seeing other failures with
an instance that includes all the contents of installcheck, so it
seems that the rest is fine.

This needs to be applied to HEAD, so I am adding an open item.

Any objections to the attached?
--
Michael