The convert operation only write an output value if the conversion
succeeded. Thus, we always initialize the buffer before calling the
conversion function.

R=ahaas@chromium.org

Change-Id: Ide230a1e608205f9067349db08adde6a90b31d6f
Reviewed-on: https://chromium-review.googlesource.com/1021377
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52703}