crypto: algif_aead - fix uninitialized variable warning

In case the user provided insufficient data, the code may return
prematurely without any operation. In this case, the processed
data indicated with outlen is zero.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>