ima: Fix a potential NULL pointer access in ima_restore_measurement_list

In restore_template_fmt, when kstrdup fails, a non-NULL value will still be
returned, which causes a NULL pointer access in template_desc_init_fields.

Fixes: c7d09367

 ("ima: support restoring multiple template formats")
Cc: stable@kernel.org
Co-developed-by: Jiaming Li <lijiaming30@huawei.com>
Signed-off-by: Jiaming Li <lijiaming30@huawei.com>
Signed-off-by: Huaxin Lu <luhuaxin1@huawei.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>