KEYS: encrypted: Add check for strsep

Add check for strsep() in order to transfer the error.

Fixes: cd3bc044

 ("KEYS: encrypted: Instantiate key with user-provided decrypted data")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>