Fix onc_to_shill translation logic error

Fix ONC logic error

A bug in onc_translator_onc_to_shill.cc is triggering a DCHECK when
a property is set in a previously non-existent dictionary.

This is a trivial fix to code that is subtle but clearly incorrect
by inspection.

BUG=none

Committed: https://crrev.com/4081b158bd2a8984e0ba3a6457998fd35c72e070
Cr-Commit-Position: refs/heads/master@{#311357}

[stevenjb, 2015-01-13 21:37:41]

stevenjb@chromium.org changed reviewers:
+ armansito@chromium.org, pneubeck@chromium.org

[armansito, 2015-01-13 21:43:44]

lgtm

[stevenjb, 2015-01-13 22:24:43]

The CQ bit was checked by stevenjb@chromium.org

[commit-bot: I haz the power, 2015-01-13 22:25:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/838043003/1

[commit-bot: I haz the power, 2015-01-13 23:07:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-13 23:07:49]

Patchset 1 (id:??) landed as
https://crrev.com/4081b158bd2a8984e0ba3a6457998fd35c72e070
Cr-Commit-Position: refs/heads/master@{#311357}