Fix failure with pgperlcritic from the TAP test of synchronous replication

Oversight in 7d81bdc8, which introduced a new routine in perl lacking a
return clause.  Per buildfarm member crake.

Backpatch down to 9.6 like its parent.

Reported-by: Andrew Dunstan
Discussion: https://postgr.es/m/16da29fa-d504-1380-7095-40de586dc038@2ndQuadrant.com
Backpatch-through: 9.6

src/test/recovery/t/007_sync_rep.pl

@@ -42,6 +42,7 @@ sub start_standby_and_wait
 
 	print("### Waiting for standby \"$standby_name\" on \"$master_name\"\n");
 	$master->poll_query_until('postgres', $query);
+	return;
 }
 
 # Initialize master node