lnwire: when reading node aliases, properly check validity

In this commit, we ensure that when we read node aliases from the wire,
we ensure that they're valid. Before this commit, we would read the raw
bytes without checking for validity which could result in us writing in
invalid node alias to disk. We've fixed this, and also updated the
quickcheck tests to generate valid strings.

lnwire/lnwire.go

@@ -76,6 +76,11 @@ func (a addressType) AddrLen() uint16 {
 // serialization.
 func WriteElement(w io.Writer, element interface{}) error {
 	switch e := element.(type) {
+	case NodeAlias:
+		if _, err := w.Write(e[:]); err != nil {
+			return err
+		}
+
 	case ShortChanIDEncoding:
 		var b [1]byte
 		b[0] = uint8(e)
@@ -429,6 +434,18 @@ func WriteElements(w io.Writer, elements ...interface{}) error {
 func ReadElement(r io.Reader, element interface{}) error {
 	var err error
 	switch e := element.(type) {
+	case *NodeAlias:
+		var a [32]byte
+		if _, err := io.ReadFull(r, a[:]); err != nil {
+			return err
+		}
+
+		alias, err := NewNodeAlias(string(a[:]))
+		if err != nil {
+			return err
+		}
+
+		*e = alias
 	case *ShortChanIDEncoding:
 		var b [1]uint8
 		if _, err := r.Read(b[:]); err != nil {