message: fix an overly pessimistic size calculation
diff: @@ -193,7 +193,7 @@ class Builder {
  // Reserves len bytes at pos of the current state (top of stack)
  // or throws an exception
  - if (_pos + len < _bufferPtr->size()) {
  + if (_pos + len < _bufferPtr->capacity()) {
  return; // All OK, we can just increase tos->pos by len
  }
  