ossl: Fix some bugs in the fallback AES-GCM implementation

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: 9b1d87286c78 ("ossl: Add a fallback AES-GCM implementation using AES-NI")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42838

(cherry picked from commit 47d767dab54895f3ba8abac6ab2295797394659e)