crypto: morus640 - Fix out-of-bounds access

We must load the block from the temporary variable here, not directly
from the input.

Also add forgotten zeroing-out of the uninitialized part of the
temporary block (as is done correctly in morus1280.c).

Fixes: 396be41f ("crypto: morus - Add generic MORUS AEAD implementations")
Reported-by:  <syzbot+1fafa9c4cf42df33f716@syzkaller.appspotmail.com>
Reported-by:  <syzbot+d82643ba80bf6937cd44@syzkaller.appspotmail.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>