fix handling NaN

encode.go

@@ -63,10 +63,13 @@ func (s *encodeState) encodeInteger(v int64) error {
 
 // encodeDecimal serializes an decimal according to RFC 8941 Section 4.1.5.
 func (s *encodeState) encodeDecimal(v float64) error {
-	i := int64(math.RoundToEven(v * 1000))
-	if i > MaxInteger || i < MinInteger {
+	if math.IsNaN(v) || math.IsInf(v, 0) {
+		return fmt.Errorf("sfv: decimal %f is not a finite number", v)
+	}
+	if v > MaxDecimal || v < MinDecimal {
 		return fmt.Errorf("sfv: decimal %f is out of range", v)
 	}
+	i := int64(math.RoundToEven(v * 1000))
 
 	// write the sign
 	if i < 0 {

encode_test.go

@@ -442,6 +442,14 @@ func TestEncode_invalidTypes(t *testing.T) {
 		t.Error("want error, not not")
 	}
 
+	// NaN
+	_, err = EncodeItem(Item{
+		Value: math.NaN(),
+	})
+	if err == nil {
+		t.Error("want error, not not")
+	}
+
 	// in Parameters
 	_, err = EncodeItem(Item{
 		Value: 1,