plugin/rewrite: prevent illegal names (#1972)

Log and returns an error when the name rewrite creates a name that is
illegal. Add test in name_test.go to see if an error is returned.

Possible followup could be the only check this if a name-rewrite is
done.

Fixes: #1638

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/rewrite/setup.go

@@ -3,10 +3,13 @@ package rewrite
 import (
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
+	clog "github.com/coredns/coredns/plugin/pkg/log"
 
 	"github.com/mholt/caddy"
 )
 
+var log = clog.NewWithPlugin("rewrite")
+
 func init() {
 	caddy.RegisterPlugin("rewrite", caddy.Plugin{
 		ServerType: "dns",