mkfs.ubifs: Initialize 'cipher_name' as NULL

mkfs.ubifs: Initialize 'cipher_name' as NULL

Variable 'cipher_name' is defined on stack without initialization, when
an user invokes mkfs with '--key' and without '-C', 'cipher_name' is a
random value, which could cause init_fscrypt_context() failed to find a
valid cipher.
Fix it by initializing 'cipher_name' as NULL when it is declared.

Fixes: cc4c5e295f546 ("mkfs.ubifs: Enable support for building without crypto")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>