Place assertions for handles after init check

This lets automated testing check that GLFW_NOT_INITIALIZED is emitted
for every public function.

src/x11_window.c

@@ -3302,9 +3302,6 @@ GLFWAPI Display* glfwGetX11Display(void)
 
 GLFWAPI Window glfwGetX11Window(GLFWwindow* handle)
 {
-    _GLFWwindow* window = (_GLFWwindow*) handle;
-    assert(window != NULL);
-
     _GLFW_REQUIRE_INIT_OR_RETURN(None);
 
     if (_glfw.platform.platformID != GLFW_PLATFORM_X11)
@@ -3313,6 +3310,9 @@ GLFWAPI Window glfwGetX11Window(GLFWwindow* handle)
         return None;
     }
 
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     return window->x11.handle;
 }