Fix Index out of range in .ToDisplayString()

csharp/Lib/Units/Unit.cs

@@ -19,12 +19,12 @@ public abstract class Unit
 
         var i = 8;
 
-        while (a >= 10000)
+        while (a >= 10000 && i < MaxPrefix)
         {
             a /= 1000;
             i++;
         }
-        while (a < 10)
+        while (a < 10 && i > 0)
         {
             a *= 1000;
             i--;
@@ -38,5 +38,7 @@ public abstract class Unit
     }
 
     private static readonly IReadOnlyList<String> Prefix = new[] { "y", "z", "a", "f", "p", "n", "µ", "m", "", "k", "M", "G", "T", "P", "E", "Y" };
+
+    private static Int32 MaxPrefix { get; } = Prefix.Count - 1;
 }