[WPF] Fix FormsPanel MeasureOverride negative requested size (#10653)

* Update FormsPanel.cs

* Update FormsPanel.cs

* Update FormsPanel.cs

* fix

Co-authored-by: Samantha Houts <[email protected]>

Author: tessarolli

Xamarin.Forms.Platform.WPF/FormsPanel.cs

@@ -79,11 +79,11 @@ protected override System.Windows.Size MeasureOverride(System.Windows.Size avail
 			if (double.IsInfinity(elementDesiredWidth) || double.IsPositiveInfinity(elementDesiredHeight))
 			{
 				Size request = Element.Measure(elementDesiredWidth, elementDesiredHeight, MeasureFlags.IncludeMargins).Request;
-				
-				if (request.Width == -1)
+        
+				if (request.Width < 0)
 					request.Width = 0.0;
 
-				if (request.Height == -1)
+				if (request.Height < 0)
 					request.Height = 0.0;
 
 				result = new System.Windows.Size(request.Width, request.Height);