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Due to its OpenSSL-based origins, ECDH_compute_key returns the output
length or -1 for error, unlike standard BoringSSL functions which are
0/1. This CL fixes the pattern of checking the return value.

(Doesn't matter in practice because it doesn't fail unless the private
key is on the wrong group.)

Change-Id: I87ceb4afa419ae578eed9cbf38edebe7e061570a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159749
Commit-Queue: Adam Langley <agl@chromium.org>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#761162}