Fix for pg_upgrade

The attached patches fixes a pg_upgrade crash in 9.0 caused by a new
cluster database that doesn't exist in the old cluster; instead throw
an error. This was reported to me by EnterpriseDB testing staff. This
bug does not exist in git head.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +