Fix ONC 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.

TBR=pneubeck@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/838043003

Cr-Commit-Position: refs/heads/master@{#311357}
(cherry picked from commit 4081b158bd2a8984e0ba3a6457998fd35c72e070)

Review URL: https://codereview.chromium.org/870793002

Cr-Commit-Position: 665a5787e4610f1ec55d3a4e2f02ab0724e38987@{#87}
Cr-Branched-From: 4cd5f18d847990599feba509541d8a7b9a16883a@{#310958}

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