crypto: skcipher - Fix crash on zero-length input

crypto: skcipher - Fix crash on zero-length input

commit 0cabf2af6f5ac3c88cb106c4e06087a5a39b8e1e upstream.

The skcipher walk interface doesn't handle zero-length input
properly as the old blkcipher walk interface did.  This is due
to the fact that the length check is done too late.

This patch moves the length check forward so that it does the
right thing.

Fixes: b286d8b1a690 ("crypto: skcipher - Add skcipher walk...")
Reported-by: Stephan Müller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>