lnwire: add fuzz target for Fee TLV

The new Fee TLV is not included in any other messages within the lnwire
package, so it currently has no fuzzing coverage.  This fuzz target
directly tests the encoding/decoding of the TLV to get some coverage.

lnwire/fuzz_test.go

@@ -1053,3 +1053,30 @@ func FuzzClosingComplete(f *testing.F) {
 		harness(t, data)
 	})
 }
+
+// FuzzFee tests that decoding and re-encoding a Fee TLV does not mutate it.
+func FuzzFee(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data []byte) {
+		if len(data) > 8 {
+			return
+		}
+
+		var fee Fee
+		var buf [8]byte
+		r := bytes.NewReader(data)
+
+		if err := feeDecoder(r, &fee, &buf, 8); err != nil {
+			return
+		}
+
+		var b bytes.Buffer
+		require.NoError(t, feeEncoder(&b, &fee, &buf))
+
+		// Use bytes.Equal instead of require.Equal so that nil and
+		// empty slices are considered equal.
+		require.True(
+			t, bytes.Equal(data, b.Bytes()), "%v != %v", data,
+			b.Bytes(),
+		)
+	})
+}