[bigint] Fix Karatsuba intermediate result length

When adding up the results of the recursive steps, the Karatsuba
algorithm can temporarily have intermediate results that are one
bit bigger than the final result. This patch makes sure we handle
that case correctly.
Since that extra bit would always get subtracted again, the old
code would not have caused incorrect results or memory corruption,
but it did run into DCHECK-failures, and potentially could have
caused segfaults.

Bug: v8:11515, chromium:1223724
Change-Id: I3592835d01cc36def8f0a9bae625e9249864ef78
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2988758


Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75509}