Only change over original: Init sig_index to 0 at
function-body-decoder-impl.h:168, to make MSAN happy on error path.

R=titzer@chromium.org

Change-Id: I9ac17215360523b656b10d2466201001b65992c0
Reviewed-on: https://chromium-review.googlesource.com/712655Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48463}