Fix encoder panic when last value is empty

Fix a panic in the encoder due to trying to write an empty value at
the end of a line that contains multi-byte characters.

buff.go

@@ -3,6 +3,7 @@ package fixedwidth
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"unicode/utf8"
 )
 
@@ -45,6 +46,11 @@ func lineBufferFromValue(value rawValue) *lineBuilder {
 
 // WriteValue writes the given value to the lineBuilder at the give start index.
 func (b *lineBuilder) WriteValue(start int, value rawValue) {
+	// If the value is empty there is nothing to write.
+	if len(value.data) == 0 {
+		return
+	}
+
 	// Fast path for ascii only operation.
 	if !b.hasMultiByteChar() && !value.hasMultiByteChar() {
 		copy(b.data[start:], value.data)

encode_test.go

@@ -242,6 +242,24 @@ func TestMarshal_backwardCompatibility(t *testing.T) {
 	})
 }
 
+func TestEncoder_regressions(t *testing.T) {
+	// Ensure the encoder doesn't panic when encoding an empty value at the end of a line
+	// that contains multi-byte characters.
+	// See: https://github.com/ianlopshire/go-fixedwidth/issues/58
+	t.Run("issue 58", func(t *testing.T) {
+		var v struct {
+			Foo string `fixed:"1,1"`
+			Bar string `fixed:"2,2,right"`
+		}
+		v.Foo = "Ç"
+
+		buf := new(bytes.Buffer)
+		e := NewEncoder(buf)
+		e.SetUseCodepointIndices(true)
+		_ = e.Encode(v)
+	})
+}
+
 func TestNewValueEncoder(t *testing.T) {
 	for _, tt := range []struct {
 		name      string