deps: backport IsValid changes from 4e8736d in V8

V8 erroneously did null pointer checks on `this`.
It can lead to a SIGSEGV crash if node is compiled with GCC 6.
Backport relevant changes from [1] that fix this issue.

[1]: https://codereview.chromium.org/1900423002

Fixes: https://github.com/nodejs/node/issues/6272
PR-URL: https://github.com/nodejs/node/pull/6544


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>