libwebp: cherry-pick VP8LBitWriter fuzzing fix

928a75de webp: Fix VP8LBitWriterClone() bug

Note this only affects code tested under the fuzzer, chrome itself does not use
threading.

BUG=chromium:917029

Change-Id: I0d94a3d8e62b247fd616eebe1009868dc8a5ed2e
Reviewed-on: https://chromium-review.googlesource.com/c/1407628
Reviewed-by: Urvang Joshi <urvang@chromium.org>
Commit-Queue: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#622196}
2 files changed