Description
When the RowCount/ColumnCount amount is decreased, and there are resized rows/column, the scrollable area get invalid.

Steps to reproduce:
  RadVirtualGrid radVirtualGrid1 = new RadVirtualGrid();

        public Form1()
        {
            InitializeComponent();

            radVirtualGrid1.Parent = this;
            radVirtualGrid1.Dock = DockStyle.Fill;
            this.Controls.SetChildIndex(radVirtualGrid1, 0);
            button1.BringToFront();

            radVirtualGrid1.RowCount = 1000;
            radVirtualGrid1.ColumnCount = 50;

            radVirtualGrid1.CellValueNeeded += radVirtualGrid1_CellValueNeeded;


            for (int i = 0; i < 50; i++)
            {
                radVirtualGrid1.MasterViewInfo.SetRowHeight(i, 30);

                radVirtualGrid1.MasterViewInfo.SetColumnWidth(i, 60);
            }
        }

        void radVirtualGrid1_CellValueNeeded(object sender, VirtualGridCellValueNeededEventArgs e)
        {
            e.Value = e.RowIndex + " " + e.ColumnIndex;
        }


        private void button1_Click(object sender, EventArgs e)
        {
            radVirtualGrid1.RowCount = 20;
            radVirtualGrid1.ColumnCount = 20;
        }