add missing PQfinish() calls to vacuumdb

I noticed that vacuum_one_database() doesn't call PQfinish() before
pg_fatal() in a few of the server version checks. I seem to have
unintentionally established this precedent in commit 00d1e88. Michael
claimed to have fixed it before committing [0], but that seems to have been
missed, too. I don't think this is a huge problem, but it does seem nicer
to properly close the connection before exiting. If there are no
objections, I plan to commit and back-patch the attached patch shortly.

[0] https://postgr.es/m/20190108020300.GH22498%40paquier.xyz

--
nathan