Fix the OpenSSL implementation of ExportKeyingMaterial.

Fix the OpenSSL implementation of ExportKeyingMaterial.

The empty context and not using a context are different.

BUG=none

Committed: https://crrev.com/866c3d4a7669272896391a35dd768aa6f685af42
Cr-Commit-Position: refs/heads/master@{#323958}

[davidben, 2015-04-03 19:45:10]

davidben@chromium.org changed reviewers:
+ rsleevi@chromium.org

[davidben, 2015-04-03 19:45:11]

Not high priority: I just noticed it last night and it took 5 minutes. No
callers currently pass non-empty contexts.

[Ryan Sleevi, 2015-04-06 03:53:26]

LGTM. Took me a while to find in RFC 5705 where this was stated, so for my own
sake

4.  Exporter Definition
   The context value
   length is encoded as an unsigned, 16-bit quantity (uint16; see
   [RFC5246], Section 4.4) representing the length of the context value.
   The context MAY be zero length.

[davidben, 2015-04-06 21:03:36]

The CQ bit was checked by davidben@chromium.org

[commit-bot: I haz the power, 2015-04-06 21:04:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1053233003/1

[commit-bot: I haz the power, 2015-04-06 21:56:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-06 21:57:50]

Patchset 1 (id:??) landed as
https://crrev.com/866c3d4a7669272896391a35dd768aa6f685af42
Cr-Commit-Position: refs/heads/master@{#323958}