Fix GCC warning about dangling reference

Fixes:

```
.../src/core/tokenizer.cpp:130:33: warning: possibly dangling reference to a temporary [-Wdangling-reference]
  130 |     const TextBreaks::Position &textBreak = this->breaks().at(itemPosition);
      |                                 ^~~~~~~~~
.../src/core/tokenizer.cpp:130:62: note: the temporary was destroyed at the end of the full expression ‘Sonnet::BreakTokenizerPrivate::breaks() const().QList<Sonnet::TextBreaks::Position>::at(((qsizetype)((Sonnet::BreakTokenizerPrivate*)this)->Sonnet::BreakTokenizerPrivate::itemPosition))’
  130 |     const TextBreaks::Position &textBreak = this->breaks().at(itemPosition);
      |                                             ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
```