crypto: af_alg - Initialize sg_num_bytes in error code path

The RX SGL in processing is already registered with the RX SGL tracking
list to support proper cleanup. The cleanup code path uses the
sg_num_bytes variable which must therefore be always initialized, even
in the error code path.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Reported-by:  <syzbot+9c251bdd09f83b92ba95@syzkaller.appspotmail.com>
#syz test: https://github.com/google/kmsan.git master
CC: <stable@vger.kernel.org> #4.14
Fixes: e870456d ("crypto: algif_skcipher - overhaul memory management")
Fixes: d887c52d

 ("crypto: algif_aead - overhaul memory management")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>