aesni: Fix an out-of-bounds read in AES_GCM_decrypt()

This is the same as 4285655adb74 ("aesni: Avoid a potential
out-of-bounds load in AES_GCM_encrypt()") except for the decryption
path.

Reported by: Jenkins (KASAN job)
Reviewed by: cem
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33474