WindowServer: Don't crash when we can't allocate a bitmap

When the client specifies an absurdly large window size
render_to_cache() fails to allocate a bitmap and crashes.

Refs #7688.

Userland/Services/WindowServer/WindowFrame.cpp

@@ -409,8 +409,14 @@ void WindowFrame::render_to_cache()
         if (s_tmp_bitmap)
             s_tmp_bitmap->unref();
         s_tmp_bitmap = Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, total_frame_rect.size(), scale).leak_ref();
+        if (!s_tmp_bitmap) {
+            dbgln("Could not create bitmap of size {}", total_frame_rect.size());
+            return;
+        }
     }
 
+    VERIFY(s_tmp_bitmap);
+
     auto top_bottom_height = total_frame_rect.height() - window_rect.height();
     auto left_right_width = total_frame_rect.width() - window_rect.width();