X11: Fix missing check for NET_WM_STATE

Closes #1356.

README.md

@@ -239,6 +239,7 @@ information on what to include when reporting a bug.
 - [X11] Bugfix: Selection I/O reported but did not support `COMPOUND_TEXT`
 - [X11] Bugfix: Latin-1 text read from selections was not converted to UTF-8
 - [X11] Bugfix: NVidia EGL would segfault if unloaded before closing the display
+- [X11] Bugfix: Checking window maximized attrib could crash some WMs (#1356)
 - [Linux] Added workaround for missing `SYN_DROPPED` in pre-2.6.39 kernel
           headers (#1196)
 - [Linux] Moved to evdev for joystick input (#906,#1005)

src/x11_window.c

@@ -2450,6 +2450,14 @@ int _glfwPlatformWindowMaximized(_GLFWwindow* window)
     Atom* states;
     unsigned long i;
     GLFWbool maximized = GLFW_FALSE;
+
+    if (!_glfw.x11.NET_WM_STATE ||
+        !_glfw.x11.NET_WM_STATE_MAXIMIZED_VERT ||
+        !_glfw.x11.NET_WM_STATE_MAXIMIZED_HORZ)
+    {
+        return maximized;
+    }
+
     const unsigned long count =
         _glfwGetWindowPropertyX11(window->x11.handle,
                                   _glfw.x11.NET_WM_STATE,