Fix resolving CNAME with no proxy (#564)

This fixes a crash when we resolve (or try to) an external CNAME
when no proxy is set.

Add test as well.

test/file_cname_proxy_test.go

@@ -0,0 +1,52 @@
+package test
+
+import (
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/coredns/coredns/middleware/proxy"
+	"github.com/coredns/coredns/middleware/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+func TestZoneExternalCNAMELookup(t *testing.T) {
+	t.Parallel()
+	log.SetOutput(ioutil.Discard)
+
+	name, rm, err := TempFile(".", exampleOrg)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	// Corefile with for example without proxy section.
+	corefile := `example.org:0 {
+       file ` + name + `
+}
+`
+	i, err := CoreDNSServer(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+
+	udp, _ := CoreDNSServerPorts(i, 0)
+	if udp == "" {
+		t.Fatalf("Could not get UDP listening port")
+	}
+	defer i.Stop()
+
+	p := proxy.NewLookup([]string{udp})
+	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
+
+	resp, err := p.Lookup(state, "cname.example.org.", dns.TypeA)
+	if err != nil {
+		t.Fatalf("Expected to receive reply, but didn't: %s", err)
+	}
+	// There should only be a CNAME in the answer section.
+	if len(resp.Answer) != 1 {
+		t.Fatalf("Expected 1 RR in answer section got %d", len(resp.Answer))
+	}
+}