This closes #1384, fix segmentation fault in `formattedValue` (#1385)

- Add nil pointer guard in cell format
- Add tests to verify the nil checks in formattedValue

Co-authored-by: Zach Clark <zachmclark@gmail.com>

cell.go

@@ -1292,6 +1292,9 @@ func (f *File) formattedValue(s int, v string, raw bool) string {
 		return v
 	}
 	styleSheet := f.stylesReader()
+	if styleSheet.CellXfs == nil {
+		return v
+	}
 	if s >= len(styleSheet.CellXfs.Xf) {
 		return v
 	}
@@ -1306,7 +1309,7 @@ func (f *File) formattedValue(s int, v string, raw bool) string {
 	if ok := builtInNumFmtFunc[numFmtID]; ok != nil {
 		return ok(v, builtInNumFmt[numFmtID], date1904)
 	}
-	if styleSheet == nil || styleSheet.NumFmts == nil {
+	if styleSheet.NumFmts == nil {
 		return v
 	}
 	for _, xlsxFmt := range styleSheet.NumFmts.NumFmt {

cell_test.go

@@ -744,6 +744,35 @@ func TestFormattedValue2(t *testing.T) {
 	}
 }
 
+func TestFormattedValueNilXfs(t *testing.T) {
+	// Set the CellXfs to nil and verify that the formattedValue function does not crash.
+	f := NewFile()
+	f.Styles.CellXfs = nil
+	assert.Equal(t, "43528", f.formattedValue(3, "43528", false))
+}
+
+func TestFormattedValueNilNumFmts(t *testing.T) {
+	// Set the NumFmts value to nil and verify that the formattedValue function does not crash.
+	f := NewFile()
+	f.Styles.NumFmts = nil
+	assert.Equal(t, "43528", f.formattedValue(3, "43528", false))
+}
+
+func TestFormattedValueNilWorkbook(t *testing.T) {
+	// Set the Workbook value to nil and verify that the formattedValue function does not crash.
+	f := NewFile()
+	f.WorkBook = nil
+	assert.Equal(t, "43528", f.formattedValue(3, "43528", false))
+}
+
+func TestFormattedValueNilWorkbookPr(t *testing.T) {
+	// Set the WorkBook.WorkbookPr value to nil and verify that the formattedValue function does not
+	// crash.
+	f := NewFile()
+	f.WorkBook.WorkbookPr = nil
+	assert.Equal(t, "43528", f.formattedValue(3, "43528", false))
+}
+
 func TestSharedStringsError(t *testing.T) {
 	f, err := OpenFile(filepath.Join("test", "Book1.xlsx"), Options{UnzipXMLSizeLimit: 128})
 	assert.NoError(t, err)