CIFS: Fix memory over bound bug in cifs_parse_mount_options

While password processing we can get out of options array bound if
the next character after array is delimiter. The patch adds a check
if we reach the end.

Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>