Fix DataTable crash on resize when columns don't fit

[pcfist]

DataTable sometimes crashes on resize, typically when it gets narrow enough for its columns not to fit. Crash happens in MeasureOverride because of negative value passed to Size ctor.

The easiest way to reproduce is to set DataTable's HorizontalAlignment to Left to disable "stretching" behavior. The following code snippet may be used (slightly modified version of DataTableSample.xaml):

<ListView ItemsSource="{x:Bind InventoryItems}" HorizontalAlignment="Left">
    <ListView.Header>
        <controls:DataTable Margin="12,0,0,0" HorizontalAlignment="Left">
            <controls:DataColumn Content="Id" />
            <controls:DataColumn CanResize="True" Content="Name" />
            <!--  Each column can be text or quickly customized by containing any content or restyled of course  -->
            <controls:DataColumn>
                <TextBlock FontWeight="SemiBold" Text="Description" />
            </controls:DataColumn>
            <controls:DataColumn Content="Quantity"/>
        </controls:DataTable>
    </ListView.Header>
    <ListView.ItemTemplate>
        <DataTemplate x:DataType="local:InventoryItem">
            <controls:DataRow HorizontalAlignment="Left">
                <TextBlock VerticalAlignment="Center" Text="{x:Bind Id}" />
                <TextBlock VerticalAlignment="Center" Text="{x:Bind Name}" />
                <TextBlock VerticalAlignment="Center" Text="{x:Bind Description}" />
                <TextBlock VerticalAlignment="Center" Text="{x:Bind Quantity}" />
            </controls:DataRow>
        </DataTemplate>
    </ListView.ItemTemplate>
    <ListView.ItemContainerStyle>
        <Style BasedOn="{StaticResource DefaultListViewItemStyle}"
                TargetType="ListViewItem">
            <Setter Property="HorizontalContentAlignment" Value="Stretch" />
        </Style>
    </ListView.ItemContainerStyle>
</ListView>

This PR adds a check to make sure width value used in MeasureOverride() are non-negative.