The behaviour was clarified in the spec:
https://github.com/WebAssembly/exception-handling/pull/97

br_on_exn (and also rethrow, which will be added in another CL) should
trap on nullptr. This CL implements this by an explicit check on each
br_on_exn (within {GetExceptionTag}). This check will be redundant if
several br_on_exn follow each other. Since also the runtime call for
{GetExceptionTag} is redundant, and also the fact that we do a runtime
call is suboptimal, I consider the whole implementation prototypical for
now anyway.

R=jkummerow@chromium.org
CC=aheejin@chromium.org

Bug: v8:10128
Change-Id: I234c3183f93fe0884aadd2ab6dbd6c2b7a07c660
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2113381
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66826}