src: fix handle leak in Buffer::New()

Fix handle leaks in Buffer::New() and Buffer::Copy() by creating the
handle scope before looking up the env with Environment::GetCurrent().

Environment::GetCurrent() calls v8::Isolate::GetCurrentContext(), which
creates a handle in the current scope, i.e., the scope created by the
caller of Buffer::New() or Buffer::Copy().

PR-URL: nodejs#7711
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

src/node_buffer.cc

@@ -298,8 +298,8 @@ MaybeLocal<Object> New(Environment* env, size_t length) {
 
 
 MaybeLocal<Object> Copy(Isolate* isolate, const char* data, size_t length) {
+  EscapableHandleScope handle_scope(isolate);
   Environment* env = Environment::GetCurrent(isolate);
-  EscapableHandleScope handle_scope(env->isolate());
   Local<Object> obj;
   if (Buffer::Copy(env, data, length).ToLocal(&obj))
     return handle_scope.Escape(obj);
@@ -348,8 +348,8 @@ MaybeLocal<Object> New(Isolate* isolate,
                        size_t length,
                        FreeCallback callback,
                        void* hint) {
+  EscapableHandleScope handle_scope(isolate);
   Environment* env = Environment::GetCurrent(isolate);
-  EscapableHandleScope handle_scope(env->isolate());
   Local<Object> obj;
   if (Buffer::New(env, data, length, callback, hint).ToLocal(&obj))
     return handle_scope.Escape(obj);
@@ -382,8 +382,8 @@ MaybeLocal<Object> New(Environment* env,
 
 
 MaybeLocal<Object> New(Isolate* isolate, char* data, size_t length) {
+  EscapableHandleScope handle_scope(isolate);
   Environment* env = Environment::GetCurrent(isolate);
-  EscapableHandleScope handle_scope(env->isolate());
   Local<Object> obj;
   if (Buffer::New(env, data, length).ToLocal(&obj))
     return handle_scope.Escape(obj);