If passing <nullptr, 0> to the decoder and trying to decode something,
it correctly detects the error and sets an error message, but still
returns true on ok(), and returns a valid result.
I triggered this error by passing a null Vector, returned by FindSection(), to
the decoder.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2410913002
Cr-Commit-Position: refs/heads/master@{#40204}