BUG #14242: Role with a setconfig "role" setting to a nonexistent role causes pg_upgrade to fail

The following bug has been logged on the website:

Bug reference:      14242
Logged by:          Nathan Bossart
Email address:      bossartn@amazon.com
PostgreSQL version: 9.5.2
Operating system:   64-bit Linux
Description:        

It is possible to modify the "role" setting in setconfig in the
pg_db_role_setting table such that it points to a nonexistent role.  When
this is the case, restoring the output of pg_dumpall will fail due to the
missing role.
 
Steps to reproduce:
 
1. As superuser, execute "create role foo with login password 'test'"
2. As foo, execute "alter role foo set role = 'foo'"
3. As superuser, execute "alter role foo rename to bar"
        a. At this point, the setconfig entry in pg_db_role_setting for
'bar' will contain '{role=foo}', which no longer exists
4. Execute pg_upgrade with the recommended steps in
https://www.postgresql.org/docs/current/static/pgupgrade.html
 
During pg_upgrade (more specifically, during the restore of the output from
pg_dumpall), the "ALTER ROLE "bar" SET "role" TO 'foo'" command generated
will fail with "ERROR: role "foo" does not exist".
 
This issue was identified by Jordan Lange and Nathan Bossart.