n-api: keep napi_env alive while it has finalizers

Manage the napi_env refcount from Finalizer instances, as the
finalizer may refer to the napi_env until it is deleted.

Fixes: nodejs#31134
Fixes: node-ffi-napi/node-ffi-napi#48

Author: addaleax

src/js_native_api_v8.h

@@ -195,9 +195,12 @@ class Finalizer {
       _finalize_callback(finalize_callback),
       _finalize_data(finalize_data),
       _finalize_hint(finalize_hint) {
+    _env->Ref();
   }
 
-  ~Finalizer() = default;
+  ~Finalizer() {
+    _env->Unref();
+  }
 
  public:
   static Finalizer* New(napi_env env,

test/node-api/test_buffer/test-external-buffer.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const common = require('../../common');
+const binding = require(`./build/${common.buildType}/test_buffer`);
+const assert = require('assert');
+
+// Regresion test for https://github.com/nodejs/node/issues/31134:
+// Buffers with finalizers are allowed to remain alive until
+// Environment cleanup without crashing the process.
+// The test stores the buffer on `process` to make sure it lives as long as
+// the Context does.
+
+process.externalBuffer = binding.newExternalBuffer();
+assert.strictEqual(process.externalBuffer.toString(), binding.theText);