Reject invalid databases in pg_get_database_ddl()

Reject invalid databases in pg_get_database_ddl()

An invalid database has datconnlimit set to -2.  pg_get_database_ddl()
emits this verbatim as CONNECTION LIMIT = -2, which ALTER DATABASE
rejects.  Error out early instead.

Reported-by: Lakshmi N <lakshmin.jhs@gmail.com>
Author: Lakshmi N <lakshmin.jhs@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Hu Xunqi <huxunqi.08@gmail.com>
Discussion: https://postgr.es/m/CA+3i_M8m1k2gFch+tU0JmAQh9FRV+pFrfTXDrJo+BqmwsTmOhg@mail.gmail.com