add handlescopes to fix potential memory leaks.

Author: chjj

src/common.h

@@ -13,6 +13,7 @@ namespace leveldown {
 NAN_INLINE bool BooleanOptionValue(v8::Local<v8::Object> options,
                                    const char* _key,
                                    bool def = false) {
+  Nan::HandleScope scope;
   v8::Local<v8::String> key = Nan::New(_key).ToLocalChecked();
   return !options.IsEmpty()
     && options->Has(key)
@@ -23,6 +24,7 @@ NAN_INLINE bool BooleanOptionValue(v8::Local<v8::Object> options,
 NAN_INLINE uint32_t UInt32OptionValue(v8::Local<v8::Object> options,
                                       const char* _key,
                                       uint32_t def) {
+  Nan::HandleScope scope;
   v8::Local<v8::String> key = Nan::New(_key).ToLocalChecked();
   return !options.IsEmpty()
     && options->Has(key)

src/iterator_async.cc

@@ -33,6 +33,7 @@ void NextWorker::Execute () {
 }
 
 void NextWorker::HandleOKCallback () {
+  Nan::HandleScope scope;
   size_t idx = 0;
 
   size_t arraySize = result.size() * 2;