Fix panic on a row whose cells are not in ascending column order

rows.go

@@ -937,9 +937,19 @@ func (ws *xlsxWorksheet) checkRow() error {
 			}
 			rowData.C[idx].R, _ = CoordinatesToCellName(rCount, rowIdx+1)
 		}
-		lastCol, _, err := CellNameToCoordinates(rowData.C[colCount-1].R)
-		if err != nil {
-			return err
+		// Cells are not guaranteed to be in ascending column order, so the
+		// highest column in the row is not necessarily the last element. Take
+		// the maximum, otherwise a cell that sorts later than the final element
+		// is scattered past the end of the rebuilt slice below.
+		var lastCol int
+		for cellIdx := range rowData.C {
+			colNum, _, err := CellNameToCoordinates(rowData.C[cellIdx].R)
+			if err != nil {
+				return err
+			}
+			if colNum > lastCol {
+				lastCol = colNum
+			}
 		}
 
 		if colCount < lastCol {

rows_test.go

@@ -1267,3 +1267,31 @@ func trimSliceSpace(s []string) []string {
 	}
 	return s
 }
+
+func TestCheckRowOutOfOrderColumns(t *testing.T) {
+	// Cells are not always stored in ascending column order. Here Z1 precedes
+	// C1, so the highest column is not the last element and sizing the rebuilt
+	// slice from that element leaves Z1 without a slot.
+	ws := &xlsxWorksheet{}
+	ws.SheetData.Row = []xlsxRow{{
+		R: 1,
+		C: []xlsxC{{R: "Z1", V: "z"}, {R: "C1", V: "c"}},
+	}}
+	assert.NoError(t, ws.checkRow())
+	assert.Len(t, ws.SheetData.Row[0].C, 26)
+	assert.Equal(t, "z", ws.SheetData.Row[0].C[25].V)
+	assert.Equal(t, "c", ws.SheetData.Row[0].C[2].V)
+
+	// Reading such a sheet through the public API must not panic.
+	f := NewFile()
+	f.Sheet.Store("xl/worksheets/sheet1.xml", &xlsxWorksheet{
+		SheetData: xlsxSheetData{Row: []xlsxRow{{
+			R: 1,
+			C: []xlsxC{{R: "Z1", T: "str", V: "z"}, {R: "C1", T: "str", V: "c"}},
+		}}},
+	})
+	value, err := f.GetCellValue("Sheet1", "Z1")
+	assert.NoError(t, err)
+	assert.Equal(t, "z", value)
+	assert.NoError(t, f.Close())
+}