crypto: skcipher - avoid NULL dereference

Stanse found a potential NULL dereference in ablkcipher_next_slow.
Even though kmalloc fails, its retval is dereferenced later. Return
from that function properly earlier.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>