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>