This closes #1432, fix panic formattedValue when style is negative (#1433)

cell.go

@@ -1319,7 +1319,7 @@ func (f *File) formattedValue(s int, v string, raw bool) (string, error) {
 	if styleSheet.CellXfs == nil {
 		return v, err
 	}
-	if s >= len(styleSheet.CellXfs.Xf) {
+	if s >= len(styleSheet.CellXfs.Xf) || s < 0 {
 		return v, err
 	}
 	var numFmtID int

cell_test.go

@@ -2,6 +2,7 @@ package excelize
 
 import (
 	"fmt"
+	_ "image/jpeg"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -11,8 +12,6 @@ import (
 	"testing"
 	"time"
 
-	_ "image/jpeg"
-
 	"github.com/stretchr/testify/assert"
 )
 
@@ -755,10 +754,16 @@ func TestFormattedValue(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, "43528", result)
 
+	// S is too large
 	result, err = f.formattedValue(15, "43528", false)
 	assert.NoError(t, err)
 	assert.Equal(t, "43528", result)
 
+	// S is too small
+	result, err = f.formattedValue(-15, "43528", false)
+	assert.NoError(t, err)
+	assert.Equal(t, "43528", result)
+
 	result, err = f.formattedValue(1, "43528", false)
 	assert.NoError(t, err)
 	assert.Equal(t, "43528", result)