After decoding an invalid function name (e.g. OOB), we stored the parsed
offset and length into the WasmFunction anyway, resulting in a runtime
CHECK failure later on.
This CL fixes this, and adds a regression test.

R=titzer@chromium.org
CC=mtrofin@chromium.org, bradnelson@chromium.org
BUG=chromium:684858

Review-Url: https://codereview.chromium.org/2656713003
Cr-Commit-Position: refs/heads/master@{#42654}