Fix assorted memory leaks.

Per Coverity (not that any of these are so non-obvious that they should not
have been caught before commit).  The extent of leakage is probably minor
to unnoticeable, but a leak is a leak.  Back-patch as necessary.

Michael Paquier

src/bin/pg_basebackup/streamutil.c

@@ -414,6 +414,7 @@ DropReplicationSlot(PGconn *conn, const char *slot_name)
 		return false;
 	}
 
+	destroyPQExpBuffer(query);
 	PQclear(res);
 	return true;
 }