Fix garbage after name in compression map

When packDomainName is called with an escaped domain name and compress
being true, bs wasn't be truncated to the correct length and would
include garbage that would be included in the compression map.

msg.go

@@ -265,7 +265,7 @@ loop:
 			}
 
 			if compress && !bsFresh {
-				roBs = string(bs)
+				roBs = string(bs[:ls])
 				bsFresh = true
 			}
 

msg_test.go

@@ -213,6 +213,28 @@ func TestUnpackDomainName(t *testing.T) {
 	}
 }
 
+func TestPackDomainNameCompressionMap(t *testing.T) {
+	msg := make([]byte, 256)
+	compression := make(map[string]int)
+
+	_, err := PackDomainName(`www\.this.is.\131an.example.org.`, msg, 0, compression, true)
+	if err != nil {
+		t.Fatalf("PackDomainName failed: %v", err)
+	}
+
+	for _, dname := range []string{
+		`www.this.is.\131an.example.org.`,
+		`is.\131an.example.org.`,
+		"\x83an.example.org.",
+		`example.org.`,
+		`org.`,
+	} {
+		if _, ok := compression[dname]; !ok {
+			t.Errorf("expected to find %q in compression map", dname)
+		}
+	}
+}
+
 func TestPackDomainNameNSECTypeBitmap(t *testing.T) {
 	ownername := "some-very-long-ownername.com."
 	msg := &Msg{