Add checks on data length for A and AAAA records (#919)

* Add checks on data length for A and AAAA records

Fixes panic when parsing A or AAAA records with no data

* Add tests Field() on empty A/AAAA data

* Refactor format test

* Add return value check on format test

format.go

@@ -31,6 +31,9 @@ func Field(r RR, i int) string {
 		switch reflect.ValueOf(r).Elem().Type().Field(i).Tag {
 		case `dns:"a"`:
 			// TODO(miek): Hmm store this as 16 bytes
+			if d.Len() < net.IPv4len {
+				return ""
+			}
 			if d.Len() < net.IPv6len {
 				return net.IPv4(byte(d.Index(0).Uint()),
 					byte(d.Index(1).Uint()),
@@ -42,6 +45,9 @@ func Field(r RR, i int) string {
 				byte(d.Index(14).Uint()),
 				byte(d.Index(15).Uint())).String()
 		case `dns:"aaaa"`:
+			if d.Len() < net.IPv6len {
+				return ""
+			}
 			return net.IP{
 				byte(d.Index(0).Uint()),
 				byte(d.Index(1).Uint()),

format_test.go

@@ -0,0 +1,16 @@
+package dns
+
+import (
+	"testing"
+)
+
+func TestFieldEmptyAOrAAAAData(t *testing.T) {
+	res := Field(new(A), 1)
+	if res != "" {
+		t.Errorf("expected empty string but got %v", res)
+	}
+	res = Field(new(AAAA), 1)
+	if res != "" {
+		t.Errorf("expected empty string but got %v", res)
+	}
+}