bn/asm/rsaz-avx2.pl: fix digit correction bug in rsaz_1024_mul_avx2.
Credit to OSS-Fuzz for finding this. CVE-2017-3738 (Imported from upstream's 5630661aecbea5fe3c4740f5fea744a1f07a6253 and 77d75993651b63e872244a3256e37967bb3c3e9e.) Confirmed with Intel SDE that the fix makes the test vector pass and that, without the fix, the test vector does not. (Well, we knew the latter already, since it was our test vector.) (cherry-picked from 296a61d6) Change-Id: I167aa3407ddab3b434bacbd18e099c55aa40ac4c Reviewed-on: https://boringssl-review.googlesource.com/23884 Reviewed-by: Adam Langley <[email protected]> Reviewed-on: https://boringssl-review.googlesource.com/23924