Allocations with new[] must be freed with delete[].

See http://valgrind.org/docs/manual/mc-manual.html#mc-manual.rudefn

src/node_buffer.cc

@@ -176,7 +176,7 @@ void Buffer::Replace(char *data, size_t length,
   if (callback_) {
     callback_(data_, callback_hint_);
   } else if (length_) {
-    delete data_;
+    delete [] data_;
     V8::AdjustAmountOfExternalAllocatedMemory(-(sizeof(Buffer) + length_));
   }