gcm_*_aesni() are used when the AVX512 implementation is not available.
Fix two bugs which manifest when handling operations spanning multiple
segments:
- Avoid underflow when the length of the input is smaller than the
residual.
- In gcm_decrypt_aesni(), ensure that we begin the operation at the
right offset into the input and output buffers.
Reviewed by: jhb
Fixes: 9b1d87286c ("ossl: Add a fallback AES-GCM implementation using AES-NI")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42838
47d767dab5