Guard against the complete absence of styles.

cell.go

@@ -118,6 +118,8 @@ func (c *Cell) GetStyle() Style {
 
 // The number format string is returnable from a cell.
 func (c *Cell) GetNumberFormat() string {
+	if c.styles == nil { return "" }
+	if c.styles.CellXfs == nil { return "" }
 	var numberFormat string = ""
 	if c.styleIndex > -1 && c.styleIndex <= len(c.styles.CellXfs) {
 		xf := c.styles.CellXfs[c.styleIndex]