message: Fix flaky sstream test
diff: @@ -33,6 +33,8 @@ ostream::Buffer::Buffer() noexcept {
  constexpr size_t initialSize = 1024;
  buffer = (char*) malloc(initialSize);
  assert(buffer);
  + // Set the first byte to 0 as this buffer might be used as a C string.
  + buffer[0] = 0;
  curr = buffer;
  capacity = initialSize;
  size = initialSize;
  