crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec

crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec

authencesn assumes an ESP/ESN-formatted AAD. When assoclen is shorter than
the minimum expected length, crypto_authenc_esn_decrypt() can advance past
the end of the destination scatterlist and trigger a NULL pointer dereference
in scatterwalk_map_and_copy(), leading to a kernel panic (DoS).

Add a minimum AAD length check to fail fast on invalid inputs.

Fixes: 104880a6b470 ("crypto: authencesn - Convert to new AEAD interface")
Reported-By: Taeyang Lee <0wn@theori.io>
Signed-off-by: Taeyang Lee <0wn@theori.io>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>