src: add consistency check to node_platform.cc

We use the `Isolate*` pointer as the sole identifier
for a V8 Isolate. In some environments (e.g. multi-threaded),
Isolates may be destroyed and new ones created; then, it
may happen that the memory that was previously used for
one `Isolate` can be re-used for another `Isolate`
after the first one has been disposed of.

This check is a little guard against accidentally
re-using the same per-Isolate platform data structure
in such cases, i.e. making sure (to the degree to which
that is possible) that the old `Isolate*` has been properly
unregistered before one at the same memory address is added.

(It’s not 100 % foolproof because the `uv_loop_t*`
pointer value could theoretically be the same as well.)

PR-URL: https://github.com/nodejs/node/pull/21156


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>