This moves the kinv computation next to k generation and adds a check for group
size as per 186-4 B.5.2.

Change-Id: I8744080d3961bc9e29054985280fc835e3f1e25c
Reviewed-on: https://boringssl-review.googlesource.com/14944
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>