Number format read fix (#741)

* fix UT-generated file names to be ignored
* fix cell value load with invalid number format ID
* fix PR issues

.gitignore

 ~$*.xlsx
 test/Test*.xlsx
 test/Test*.xlsm
+# generated files
+test/BadEncrypt.xlsx
+test/BadWorkbook.SaveAsEmptyStruct.xlsx
+test/*.png
 *.out
 *.test
 .idea

cell.go

@@ -762,10 +762,15 @@ func (f *File) formattedValue(s int, v string) string {
 		return v
 	}
 	styleSheet := f.stylesReader()
+
 	if s >= len(styleSheet.CellXfs.Xf) {
 		return v
 	}
-	numFmtID := *styleSheet.CellXfs.Xf[s].NumFmtID
+	var numFmtID int
+	if styleSheet.CellXfs.Xf[s].NumFmtID != nil {
+		numFmtID = *styleSheet.CellXfs.Xf[s].NumFmtID
+	}
+
 	ok := builtInNumFmtFunc[numFmtID]
 	if ok != nil {
 		return ok(v, builtInNumFmt[numFmtID])

cell_test.go

@@ -301,7 +301,7 @@ func TestSetCellRichText(t *testing.T) {
 	assert.EqualError(t, f.SetCellRichText("Sheet1", "A", richTextRun), `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 }
 
-func TestFormattedValue(t *testing.T) {
+func TestFormattedValue2(t *testing.T) {
 	f := NewFile()
 	v := f.formattedValue(0, "43528")
 	assert.Equal(t, "43528", v)
@@ -320,12 +320,24 @@ func TestFormattedValue(t *testing.T) {
 	assert.Equal(t, "03/04/2019", v)
 
 	// formatted value with no built-in number format ID
-	assert.NoError(t, err)
-	f.Styles.NumFmts = nil
 	numFmtID := 5
 	f.Styles.CellXfs.Xf = append(f.Styles.CellXfs.Xf, xlsxXf{
 		NumFmtID: &numFmtID,
 	})
+	v = f.formattedValue(2, "43528")
+	assert.Equal(t, "43528", v)
+
+	// formatted value with invalid number format ID
+	f.Styles.CellXfs.Xf = append(f.Styles.CellXfs.Xf, xlsxXf{
+		NumFmtID: nil,
+	})
+	v = f.formattedValue(3, "43528")
+
+	// formatted value with empty number format
+	f.Styles.NumFmts = nil
+	f.Styles.CellXfs.Xf = append(f.Styles.CellXfs.Xf, xlsxXf{
+		NumFmtID: &numFmtID,
+	})
 	v = f.formattedValue(1, "43528")
 	assert.Equal(t, "43528", v)
 }