Add missing assertion

src/window.c

@@ -628,6 +628,8 @@ GLFWAPI void glfwRestoreWindow(GLFWwindow* handle)
 GLFWAPI void glfwMaximizeWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT();
     _glfwPlatformMaximizeWindow(window);
 }