buffer: avoid undefined behaviour

Avoid 'delete this' as it can be hazardous and/or dependent on
implementations.

PR-URL: #5494
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: targos - Michaël Zasso <mic.besace@gmail.com>

src/node_buffer.cc

@@ -139,18 +139,18 @@ CallbackInfo::~CallbackInfo() {
 
 void CallbackInfo::WeakCallback(
     const WeakCallbackInfo<CallbackInfo>& data) {
-  data.GetParameter()->WeakCallback(
+  CallbackInfo* self = data.GetParameter();
+  self->WeakCallback(
       data.GetIsolate(),
       static_cast<char*>(data.GetInternalField(kBufferInternalFieldIndex)));
+  delete self;
 }
 
 
 void CallbackInfo::WeakCallback(Isolate* isolate, char* const data) {
   callback_(data, hint_);
   int64_t change_in_bytes = -static_cast<int64_t>(sizeof(*this));
   isolate->AdjustAmountOfExternalAllocatedMemory(change_in_bytes);
-
-  delete this;
 }