src: fix what a dispose without checking

If created platform with CreatePlatform, the crash occurs because
it does not check if it was initialized to v8_platform
when DisposePlatform was called.

Refs: https://github.com/nodejs/node/pull/31260


Co-authored-by: Anna Henningsen <anna@addaleax.net>
PR-URL: https://github.com/nodejs/node/pull/30467


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>