Merge pull request #6724 from AvaloniaUI/bug/x11-segfault-on-xopendisplay-fail

[X11] Check for XOpenDisplay error _before_ trying to use display

src/Avalonia.X11/X11Platform.cs

@@ -52,11 +52,14 @@ namespace Avalonia.X11
 
             XInitThreads();
             Display = XOpenDisplay(IntPtr.Zero);
+            if (Display == IntPtr.Zero)
+                throw new Exception("XOpenDisplay failed");
             DeferredDisplay = XOpenDisplay(IntPtr.Zero);
+            if (DeferredDisplay == IntPtr.Zero)
+                throw new Exception("XOpenDisplay failed");
+                
             OrphanedWindow = XCreateSimpleWindow(Display, XDefaultRootWindow(Display), 0, 0, 1, 1, 0, IntPtr.Zero,
                 IntPtr.Zero);
-            if (Display == IntPtr.Zero)
-                throw new Exception("XOpenDisplay failed");
             XError.Init();
             
             Info = new X11Info(Display, DeferredDisplay, useXim);