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

src/bin/scripts/vacuumdb.c

@@ -424,6 +424,7 @@ vacuum_one_database(const ConnParams *cparams,
 
 	if (vacopts->min_xid_age != 0 && PQserverVersion(conn) < 90600)
 	{
+		PQfinish(conn);
 		pg_log_error("cannot use the \"%s\" option on server versions older than PostgreSQL %s",
 					 "--min-xid-age", "9.6");
 		exit(1);
@@ -431,6 +432,7 @@ vacuum_one_database(const ConnParams *cparams,
 
 	if (vacopts->min_mxid_age != 0 && PQserverVersion(conn) < 90600)
 	{
+		PQfinish(conn);
 		pg_log_error("cannot use the \"%s\" option on server versions older than PostgreSQL %s",
 					 "--min-mxid-age", "9.6");
 		exit(1);
@@ -438,6 +440,7 @@ vacuum_one_database(const ConnParams *cparams,
 
 	if (vacopts->parallel_workers >= 0 && PQserverVersion(conn) < 130000)
 	{
+		PQfinish(conn);
 		pg_log_error("cannot use the \"%s\" option on server versions older than PostgreSQL %s",
 					 "--parallel", "13");
 		exit(1);