Place assertions for handles after init check

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

src/win32_window.c

@@ -2577,9 +2577,6 @@ VkResult _glfwCreateWindowSurfaceWin32(VkInstance instance,
 
 GLFWAPI HWND glfwGetWin32Window(GLFWwindow* handle)
 {
-    _GLFWwindow* window = (_GLFWwindow*) handle;
-    assert(window != NULL);
-
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (_glfw.platform.platformID != GLFW_PLATFORM_WIN32)
@@ -2589,6 +2586,9 @@ GLFWAPI HWND glfwGetWin32Window(GLFWwindow* handle)
         return NULL;
     }
 
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     return window->win32.handle;
 }