deps: v8: fix potential segfault in profiler

This change fixes a potential segfault in the sampling heap profiler.
This landed as part of a larger change upstream [1]. This is the minimal
backport that avoids the segfault.

[1]: https://git.io/vdTYL

PR-URL: https://github.com/nodejs/node/pull/15498


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>