s390/dcss: array index 'i' is used before limits check.

Avoid out-of-bounds-read by checking count before indexing.

Signed-off-by : Ameen Ali <Ameenali023@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>