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

In the Linux kernel, the following vulnerability has been resolved:

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.