Fix stop_postmaster call in get_cluster_version. (#1497)

stop_postmaster accepts single boolean arg, yet pointer of type ClusterInfo is passed. Remove this, and call stop_postmaster with `false` argument, resulting in smart (not fast) shutdown. Also remove redundant return at the end on void function.
Introduced as oversight in e20a488.


Fixed with help of coverity report:


```
** CID 640088:       Null pointer dereferences  (REVERSE_INULL)
/src/bin/pg_upgrade/pg_upgrade.c: 374           in get_cluster_version()


_____________________________________________________________________________________________
*** CID 640088:         Null pointer dereferences  (REVERSE_INULL)
/src/bin/pg_upgrade/pg_upgrade.c: 374             in get_cluster_version()
368     	cluster->dbid = dbid;
369     
370     	PQclear(res);
371     
372     	PQfinish(conn);
373     
>>>     CID 640088:         Null pointer dereferences  (REVERSE_INULL)
>>>     Null-checking "cluster" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
374     	stop_postmaster(cluster);
375     
376     	return;
377     }
378     
379     #ifdef WIN32

 
```

Author: reshke

src/bin/pg_upgrade/pg_upgrade.c

@@ -371,9 +371,7 @@ get_cluster_version(ClusterInfo *cluster)
 
 	PQfinish(conn);
 
-	stop_postmaster(cluster);
-
-	return;
+	stop_postmaster(false);
 }
 
 #ifdef WIN32