wasi: avoid hard crash if start() is never called

[cjihrig]

Prior to this commit, calling the WebAssembly instance's _start()
export directly would hard crash the process because the memory
was never configured. This commit adds an additional check that
the memory is usable. With this commit, calling _start() directly
will still lead to WASI exiting with a non-zero exit code, but
Node does not crash.

Fixes: #33175

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31137/ 💛

[tniessen]

I didn't see this PR (or the issue) before opening #33235. Please consider it as an alternative:

• Personally, I think a non-recoverable error (at least not recoverable from WASM) is more appropriate in case a WASI module was not started properly. With this PR, it is up to the WASM module to handle the error code EINVAL, even though the WASM module likely isn't at fault.
• This PR doesn't prevent calling syscalls that don't access memory.

[cjihrig]

I'm good with the approach in #33235. I'll close this.