drm/client: fix null pointer dereference in drm_client_modeset_probe

In the Linux kernel, the following vulnerability has been resolved: drm/client: fix null pointer dereference in drm_client_modeset_probe In drm_client_modeset_probe(), the return value of drm_mode_duplicate() is assigned to modeset->mode, which will lead to a possible NULL pointer dereference on failure of drm_mode_duplicate(). Add a check to avoid npd.