src: handle fatal error when Environment is not assigned to context

Previously when an uncaught JS error is thrown before Environment was
assigned to the context (e.g. a SyntaxError in a per-context script),
it triggered an infinite recursion:

1. The error message listener `node::OnMessage()` triggered
   `node::FatalException()`
2. `node::FatalException()` attempted to get the Environment
   assigned to the context entered using `Environment::GetCurrent()`
3. `Environment::GetCurrent()` previously incorrectly accepted
   out-of-bound access with the length of the embedder data array
   as index, and called `context->GetAlignedPointerFromEmbedderData()`
4. The out-of-bound access in `GetAlignedPointerFromEmbedderData()`
   triggered a fatal error, which was handled by `node::FatalError()`
5. `node::FatalError()` called `Environment::GetCurrent()`, then
   we went back to 3.

This patch fixes the incorrect guard in 3. When
`Environment::GetCurrent()` returns nullptr (when Environment is not
yet assigned to the context) in 2, it now prints the JS stack trace
and crashes directly.

PR-URL: https://github.com/nodejs/node/pull/27236


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>