src: remove invalid ToLocalChecked in EmitBeforeExit

This call can fail, we should not use `.ToLocalChecked()` here.

In the long run, we should use variants of `EmitExit()` and
`EmitBeforeExit()` that avoid this problem by properly propagating
empty `MaybeLocal`s.

Example failure:

    21:07:17 not ok 2564 parallel/test-worker-terminate-source-map
    21:07:17   ---
    21:07:17   duration_ms: 0.385
    21:07:17   severity: crashed
    21:07:17   exitcode: -6
    21:07:17   stack: |-
    21:07:17     FATAL ERROR: v8::ToLocalChecked Empty MaybeLocal.
    21:07:17      1: 0x1012f04a5 node::Abort() (.cold.1) [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      2: 0x1000b52d9 node::Abort() [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      3: 0x1000b543f node::OnFatalError(char const*, char const*) [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      4: 0x1001ffd70 v8::V8::ToLocalEmpty() [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      5: 0x1000079b8 node::EmitBeforeExit(node::Environment*) [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      6: 0x100156fea node::worker::Worker::Run() [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      7: 0x10015a4d2 node::worker::Worker::StartThread(v8::FunctionCallbackInfo<v8::Value> const&)::$_3::__invoke(void*) [/Users/iojs/build/workspace/node-test-commit-osx/nodes/osx1015/out/Release/node]
    21:07:17      8: 0x7fff72fe7109 _pthread_start [/usr/lib/system/libsystem_pthread.dylib]
    21:07:17      9: 0x7fff72fe2b8b thread_start [/usr/lib/system/libsystem_pthread.dylib]
    21:07:17   ...

PR-URL: #35484
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Richard Lau <rlau@redhat.com>

src/api/hooks.cc

@@ -45,7 +45,9 @@ void EmitBeforeExit(Environment* env) {
   Local<Integer> exit_code;
   if (!exit_code_v->ToInteger(env->context()).ToLocal(&exit_code)) return;
 
-  ProcessEmit(env, "beforeExit", exit_code).ToLocalChecked();
+  // TODO(addaleax): Provide variants of EmitExit() and EmitBeforeExit() that
+  // actually forward empty MaybeLocal<>s (and check env->can_call_into_js()).
+  USE(ProcessEmit(env, "beforeExit", exit_code));
 }
 
 int EmitExit(Environment* env) {