plugin/secondary: Fix startup transfer failure wrong zone logged (#5085)

* avoid race
Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/secondary/setup.go

@@ -23,7 +23,8 @@ func setup(c *caddy.Controller) error {
 	}
 
 	// Add startup functions to retrieve the zone and keep it up to date.
-	for _, n := range zones.Names {
+	for i := range zones.Names {
+		n := zones.Names[i]
 		z := zones.Z[n]
 		if len(z.TransferFrom) > 0 {
 			c.OnStartup(func() error {