Fix stack overflow exception for em font-size value type

Source/DataTypes/SvgUnit.cs

@@ -180,9 +180,8 @@ namespace Svg
         private IFontDefn GetFont(ISvgRenderer renderer, SvgElement owner)
         {
             if (owner == null) return null;
-
-            var visual = owner.ParentsAndSelf.OfType<SvgVisualElement>().FirstOrDefault();
-            return visual.GetFont(renderer);
+            var visual = owner.Parents.OfType<SvgVisualElement>().FirstOrDefault();
+            return visual?.GetFont(renderer);
         }
 
         /// <summary>