This function is not exported, so we don't need the optional BN_CTX
logic. Additionally, the cleanup code can be made a bit simpler and more
idiomatic.

Change-Id: Ib326eab4813fd9de9ac1df8fdc9e470c26aff092
Reviewed-on: https://boringssl-review.googlesource.com/c/32704
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>