Reject missing database name in pg_regress and cohorts.
Writing "pg_regress --dbname= ..." led to a crash, because
we weren't expecting there to be no database name supplied.
It doesn't seem like a great idea to run regression tests
in whatever is the user's default database; so rather than
supporting this case let's explicitly reject it.
Per report from Xing Guo. Back-patch to all supported
branches.
projects / thirdparty / postgresql.git / commit
