Fix division by zero (microsoft#1366)

<!--- Provide a general summary of your changes in the Title above -->

## Description
Fix division by zero exception when the window is too narrow.

## Motivation and Context
Fixes microsoft#1365 

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

WinUIGallery/ControlPages/AnnotatedScrollBarPage.xaml.cs

@@ -8,6 +8,7 @@
 //
 //*********************************************************
 
+using System;
 using System.Collections.ObjectModel;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Controls.Primitives;
@@ -181,7 +182,7 @@ private string GetItemColor(int itemIndex)
 
         private int GetItemsPerRow()
         {
-            return (itemsRepeater == null || itemsRepeater.ActualWidth == 0) ? 1 : (int) (itemsRepeater.ActualWidth / ItemWidth);
+            return (itemsRepeater == null || itemsRepeater.ActualWidth == 0) ? 1 : (int) Math.Max(itemsRepeater.ActualWidth / ItemWidth, 1);
         }
     }
 }