buffer: fix copy() segfault with zero arguments

Buffer#copy() immediately does a ToObject() on the first argument before
it checks if it's even an Object. This causes
Object::HasIndexedPropertiesInExternalArrayData() to be run on nothing,
triggering the segfault. Instead run HasInstance() on the args Value.
Which will check if it's actually an Object, before checking if it
contains data.

Fixes: nodejs#1519

src/node_buffer.cc

@@ -303,11 +303,11 @@ void Base64Slice(const FunctionCallbackInfo<Value>& args) {
 void Copy(const FunctionCallbackInfo<Value> &args) {
   Environment* env = Environment::GetCurrent(args);
 
-  Local<Object> target = args[0]->ToObject(env->isolate());
-
-  if (!HasInstance(target))
+  if (!HasInstance(args[0]))
     return env->ThrowTypeError("first arg should be a Buffer");
 
+  Local<Object> target = args[0]->ToObject(env->isolate());
+
   ARGS_THIS(args.This())
   size_t target_length = target->GetIndexedPropertiesExternalArrayDataLength();
   char* target_data = static_cast<char*>(