vacuumdb: Add missing PQfinish() calls to vacuum_one_database().
A few of the version checks in vacuum_one_database() do not call
PQfinish() before exiting. This precedent was unintentionally
established in commit
00d1e88d36, and while it's probably not too
problematic, it seems better to properly close the connection.
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/Z6JAwqN1I8ljTuXp%40nathan
Backpatch-through: 13