Fix 007_sync_rep.pl to notice failures in ALTER SYSTEM SET.

If a test case tried to set an invalid value of synchronous_standby_names,
the test script didn't detect that, which seems like a bad idea.
Noticed while testing a proposed patch that broke some of these
test cases.

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

@@ -18,7 +18,7 @@ sub test_sync_state
 
 	if (defined($setting))
 	{
-		$self->psql('postgres',
+		$self->safe_psql('postgres',
 			"ALTER SYSTEM SET synchronous_standby_names = '$setting';");
 		$self->reload;
 	}