n-api: add missing handle scopes

Currently when building with --debug
test/addons-napi/test_threadsafe_function will error:

$  out/Debug/node test/addons-napi/test_threadsafe_function/test.js
FATAL ERROR: v8::HandleScope::CreateHandle()
  Cannot create a handle without a HandleScope
 1: 0x10004e287 node::DumpBacktrace(__sFILE*) [node/out/Debug/node]
 2: 0x1000cd37b node::Abort() [/node/out/Debug/node]
 3: 0x1000cd69f node::OnFatalError(char const*, char const*)
    [/node/out/Debug/node]
 4: 0x1004df0b1 v8::Utils::ReportApiFailure(char const*, char const*)
    [/nodejs/node/out/Debug/node]
 5: 0x100a8c0a9 v8::internal::HandleScope::Extend(
        v8::internal::Isolate*)
    [/node/out/Debug/node]
 6: 0x1004e4229 v8::EmbedderDataFor(v8::Context*,
                                    int, bool,
                                    char const*)
    [/node/out/Debug/node]
 7: 0x1004e43fa v8::Context::SlowGetAlignedPointerFromEmbedderData(int)
    [/node/out/Debug/node]
 8: 0x10001c26b v8::Context::GetAlignedP...