This fixes a bug in the offset computation when instantiating the
decoder to decode a VarInt32.
It also extends the streaming decoder test to check the error location.

R=ahaas@chromium.org

Bug: v8:8814
Change-Id: Id8ce31ce7e494cce14231febbb5b0c7d91a26e01
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505453Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60067}