cifs: fix a possible null pointer deref in decode_ascii_ssetup

When kzalloc fails, we will end up doing NULL pointer derefrence

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Steve French <smfrench@gmail.com>