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.

Cc: stable@vger.kernel.org
Fixes: cf13909a ("drm/fb-helper: Move out modeset config code")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240802044736.1570345-1-make24@iscas.ac.cn

drivers/gpu/drm/drm_client_modeset.c

@@ -880,6 +880,11 @@ int drm_client_modeset_probe(struct drm_client_dev *client, unsigned int width,
 
 			kfree(modeset->mode);
 			modeset->mode = drm_mode_duplicate(dev, mode);
+			if (!modeset->mode) {
+				ret = -ENOMEM;
+				break;
+			}
+
 			drm_connector_get(connector);
 			modeset->connectors[modeset->num_connectors++] = connector;
 			modeset->x = offset->x;